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BOTANY 

PRINCIPLES  AND  PROBLEMS 


McGRAW-HILL 

AGRICULTURAL  AND 

BIOLOGICAL  PUBLICATIONS 

Charles  V.  Piper,  Consulting  Editor 


Babcock  and  Clausen's — 

GENETICS  IN  RELATION  TO  AGRICULTURE 
Babcock  and  Collins' — 

GENETICS  LABORATORY  MANUAL 
Shull,  La  Rue  and  Ruthven's — 

PRINCIPLES  OF  ANIMAL  BIOLOGY 
Skull's— 

LABORATORY    DIRECTIONS    IN    PRINCIPLES     OF 
ANIMAL  BIOLOGY 
Thatcher's — 

CHEMISTRY  OF  PLANT  LIFE 
Hayes  ami  Garber's — 

BREEDING  CROP  PLANTS 
Sharp's — 

AN  INTRODUCTION  TO  CYTOLOGY 
Fern  aid's — 

APPLIED  ENTOMOLOGY 


Gardner,  Bradford  and  Hooker's — 

FUNDAMENTALS  OF  FRUIT  PRODUCTION 
Cruess  and  Christie's — 

LABORATORY  MANUAL  OF  FRUIT  AND  VEGETABLE 
PRODUCTS 
Piper  and  Morse's — 

THE  SOYBEAN 
Carrier — 

THE  BEGINNINGS  OF  AGRICULTURE  IN  AMERICA 
Lbhnis  and  Fred's — 

TEXTBOOK  OF  AGRICULTURAL  BACTERIOLOGY 
Thompson's — 

VEGETABLE  CROPS 

Sinnott's — 

BOTANY:  PRINCIPLES  AND  PRACTICE 


All  four  main  divisioiis  of   iln-  plaul   kiugdt 


( Frontispiece) 
1  u  represented  in  this  picture. 


BOTANY 

PRrNCIPLES  AND  PIU)F,LEMS 


BY 


EDMUND  W.  SINNOTT 

Professor  of  Botany,  Connecticut  Agricultural  College 


First  Edition 


McGRAW-HILL  BOOK  COMPANY,  Inc. 
NEW  YORK:  370  SEVENTH  AVENUE 

LONDON:  6  &  8  BOUVERIE  ST.,  E.  C.  4 

1923 


Copyright,  1923,  by  the 
McGraw-Hill  Book  Company,  Inc. 

PRINTED    IN   THE    UNITED   STATES    OF  AMERICA 


THE    MAPLE    PRESS    -    YO  I 


To  My  Mother 


PREFACE 

The  present  volume,  an  outgrowth  of  experience  in  presenting 
to  college  freshmen  a  course  in  elementary  botany,  endeavors 
to  set  forth  somewhat  briefly  and  concisely  the  more  important 
facts  concerning  the  morphology,  physiology  and  classification 
of  plants,  and  to  provide  a  body  of  problem  material  which  may 
be  of  assistance  in  stimulating  thought  and  in  promoting  class 
discussion. 

The  consideration  of  structure  and  function  in  the  earlier 
portions  of  the  book  is  confined  mainly  to  the  seed  plants,  and 
the  distinctive  characteristics  of  the  other  members  of  the  plant 
kingdom  are  discussed  in  the  last  five  chapters.  Should  the 
course  be  too  brief  to  take  up  these  groups  in  detail,  Chapter  XTII, 
which  deals  with  the  main  events  in  the  history  of  the  plant 
kingdom  and  the  important  features  of  its  various  divisions, 
may  perhaps  be  used  to  give  the  student  an  idea  of  the  kingdom 
as  a  whole.  In  view  of  the  importance  of  the  soil  in  the  life  of 
plants,  and  in  order  to  emphasize  the  fact  that  living  things 
cannot  be  understood  without  a  knowledge  of  their  environment, 
an  early  chapter  is  devoted  entirely  to  the  soil  itself.  The 
increased  interest  in  matters  pertaining  to  inheritance  has  war- 
ranted a  special  chapter  on  heredity  and  variation;  and  a  chapter 
is  also  devoted  to  organic  evolution,  emphasizing  its  botanical 
aspects.  The  chapters  have  been  so  written  as  to  be  separately 
understandable  and  may  readily  be  taken  up  in  some  other  order 
than  that  in  which  they  are  here  presented.  The  text  is  not 
primarily  designed  for  agricultural  students  but  many  of  the 
questions  naturally  involve  a  practical  apphcation  of  botanical 
principles  and  will  perhaps  commend  themselves  especially  to 
those  whose  interest  in  botany  is  chiefly  agricultural. 

The  rather  extensive  lists  of  "Questions  for  Thought  and 
Discussion",  which  are  perhaps  the  most  novel  feature  of  the 
present  volume,  have  resulted  from  an  attempt  to  stimulate 
within  the  student  an  attitude  of  interest,  of  curiosity  and  of 
critical  thought  toward  the  multitude  of  problems  which  plants 
present,  and  thus  to  provide  him  with  a  clearer  insight  into  the 
way  in  which  plants  are  constructed  and  function,  and  a  firmer 


X  PREFACE 

command  of  botanical  science  in  general,  than  can  be  given  him 
merely  through  a  series  of  lectures  and  recitations.  Anyone 
who  has  mastered  the  facts  presented  in  the  text  and  who  has 
at  his  command  a  reasonably  broad  body  of  experience,  ought 
to  be  able,  through  the  exercise  of  a  little  thought,  to  prepare 
satisfactory  answers  for  all  of  them.  "Thought  questions" 
are  by  no  means  new  in  botanical  pedagogy,  but  it  is  believed 
that  no  previous  text  has  employed  them  to  quite  such  an  extent 
as  does  the  present  one.  It  is  hoped  that  their  inclusion  here 
may  encourage  a  method  of  class-room  procedure  more  satis- 
factory than  the  common  but  somewhat  outworn  practice  where- 
in a  monopoly  of  thinking  and  talking  is  enjoyed  by  the 
instructor,  and  the  student  is  chiefly  required  to  memorize  a 
series  of  what  often  seem  to  him  to  be  unrelated  facts.  The 
"Reference  Problems"  are  designed  to  send  the  student  occasion- 
ally to  other  sources  of  information  than  his  textbook,  and  thus 
to  broaden  his  point  of  view  and  dispel  from  his  mind  the  com- 
fortable assurance  that  any  particular  authority  may  be  infallible. 

To  all  those  who  have  been  of  assistance  in  the  preparation  of 
the  book  the  author  desires  to  express  his  sincere  thanks.  Many 
of  his  colleagues  have  contributed  helpful  suggestions  and 
information  in  matters  relating  to  their  special  fields,  and  to 
Professor  G.  S.  Torrey  is  he  especially  indebted  for  advice  and 
assistance  during  the  course  of  the  work.  Professor  Torrey  and 
Dr.  L.  C.  Dunn  have  been  good  enough  to  read  and  criticize 
portions  of  the  manuscript.  A.  I.  Weinstein  has  been  helpful 
in  many  ways.  Professor  M.  L.  Fernald  of  the  Gray  Herbarium 
kindly  supplied  the  data  used  in  Figs.  131,  132  and  133.  To 
Professor  B.  M.  Davis  of  the  University  of  Michigan  and  Pro- 
fessor J.  W.  Harshberger  of  the  University  of  Pennsylvania  the 
author  is  under  obligation  for  their  courtesy  in  providing  material 
and  facilities  for  the  preparation  of  a  number  of  the  figures.  He 
is  also  much  indebted  to  his  wife  for  frequent  assistance  during 
the  preparation  of  text  and  illustrations. 

The  great  majority  of  the  illustrations  are  original.  They  are 
the  work  of  several  individuals,  to  whom  the  author  is  grateful 
for  hearty  cooperation.  Mrs.  Grace  Griffin  Hosking  is  responsi- 
ble for  Figs.  21,  22,  26,  66,  81,  99,  135,  136,  141,  142,  144,  145, 
146,  147,  149,  151,  152,  153,  157,  158,  159,  160,  161,  174,  181,  184, 
185,  187,  188,  189,  196,  and  216;  H.  C.  Creutzburg  for  Figs.  20, 
45,  47,  96,  156,  162,  166,  167,  169,  170,  175,  177,  178,  180,  182, 


PREFACE  xi 

186,  190,  191,  193,  209,  211,  and  213;  Mori  Uyehara  for  Figs. 
154  and  155;  E.  J.  Slanetz  for  Fig.  18,  and  A.  I.  Weinstein  for 
Figs.  137  and  143,  as  well  as  for  Figs.  Ill  and  230  from  his 
unpublished  work.  The  other  original  drawings,  some  ninety  in 
all,  are  by  the  author.  He  is  indebted  to  Professor  W.  F.  Ganong 
and  the  Macmillan  Co.  for  the  use  of  four  figures  from  "A 
Textbook  of  Botany  for  Colleges";  to  Professor  W.  F.  Ganong 
and  Henry  Holt  and  Co.  for  the  use  of  two  figures  from  "The 
Living  Plant",  and  to  Dr.  C.  S.  Gager  and  P.  Blakiston's  Sons 
and  Co.  for  the  use  of  four  figures  from  "The  Fundamentals  of 
Botany".  To  the  United  States  Forest  Service,  the  United 
States  Forest  Products  Laboratory,  the  Desert  Laboratory  of  the 
Carnegie  Institution,  the  Brooklyn  Botanic  Garden,  the  editors 
of  the  Journal  of  Heredity  and  of  Genetics,  Dr.  A.  J.  Grout, 
Professors  E.  B.  Babcock  and  R.  E.  Clausen,  Dr.  M.  A.  Howe, 
Professor  L  W.  Bailey,  Professor  E.  W.  Berry,  Professor  M.  L. 
Fernald,  and  Professor  G.  S.  Torrey  the  author  is  indebted  for 
original  illustrations  or  permission  to  use  published  ones.  A 
few  familiar  figures  have  also  been  taken  from  some  of  the  older 
texts. 

The  author  will  be  very  glad  to  welcome  criticisms  and  sugges- 
tions, particularly  those  which  relate  to  the  employment  of  the 
Questions  and  Problems  in  the  conduct  of  class  work. 

E.    W.    SiNNOTT. 
Connecticut  Agricultural  College, 
July,  1923. 


CONTENTS 

I'agb 

Preface ix 

To  THE  Student xvii 

CHAPTER  I 

The  Science  of  Botany 1 

The  subdivisions  of  botany — The  history  of  botany. 

CHAPTER  II 

Introductory  Survey 11 

The  plant  kingdom — The  structures  and  functions  of  plants — The 
root  and  its  functions — The  leaf  and  its  functions — The  stem  and 
its  functions — ^The  reproductive  organs  and  their  function — 
Metabolic  processes. 

CHAPTER  III 

The  Soil  and  Its  Importance  to  Plants 23 

Rock  particles — Water — Air — Organic  matter — Dissolved  sub- 
stances— Organisms. 

CHAPTER  IV 

The  Root  and  Its  Functions 40 

External  structure — The  absorbing  region — The  plant  cell — 
Internal  structure  of  roots — Diffusion  and  osmosis — Diffusion 
and  osmosis  in  the  plant  cell — The  absorption  of  Avater  and  salts — 
Other  osmotic  phenomena  in  the  plant — Other  functions  of  the 
root. 

CHAPTER  V 

The  Leaf  and  Its  Functions 63 

The  structure  of  the  leaf — Photosynthesis — Transpiration. 

CHAPTER  VI 

The  Stem  and  Its  Functions 88 

The  external  structure  of  the  stem — The  internal  structure  of  the 
stem — The  structure  of  wood — The  ascent  of  sap  in  stems — The 
translocation  of  foods. 

CHAPTER  VII 

Metabolism 117 

Plant  foods — Digestion — Assimilation — Respiration, 
xiii 


xiv  CONTENTS 

CHAPTER  VIII 

Page 

Growth 138 

The  production  of  new  cells — Growing-points  and  their  function 
— Terminal  growing-points — Lateral  growing-point  or  cambium — 
Differentiation. 

CHAPTER  IX 

The  Plant  and  Its  Environment 154 

Stimulus  and  response — Temperature — Light — Gravity — Moisture 
— Chemical  substances — Living  organisms. 

CHAPTER  X 

Reproduction 182 

Asexual  reproduction — Sexual  reproduction — Pollination — Fertili- 
zation— ^Seed  development — The  fruit — Seed  dispersal — Seed 
germination. 

CHAPTER  XI 

Heredity  and  Variation 206 

Heredity  —  Variation  —  Laws  of  inheritance  —  Inheritance  of 
acquired  characters — Mendel's  law  of  inheritance — Unit  characters 
— Dominance — Segregation — Independent  assortment — Mutation. 

CHAPTER  XII 

Evolution 229 

Evidences  for  evolution — ^Lamarck's  theory — Darwin's  theory. 
"Natural  Selection" — De  Vries's  theory — Orthogenesis. 

CHAPTER  XIII 

The  Plant  Kingdom 243 

Plants  and  animals — Forward  steps  in  plant  evolution — ^1.  The 
multicellular  plant — 2.  Differentiation — 3.  Sexual  reproduction — 
4.  Alternation  of  generations — 5.  The  invasion  of  the  land — 6. 
The  evolution  of  the  seed. — Plant  classification — Nomenclature. 

CHAPTER  XIV 

The  Thallophyta 264 

Cyanophyceae  or  blue-green  algae — Chlorophyceae  or  green  algae 
— Phaeophyceae  or  brown  algae — ^Rhodophyceae  or  red  algae — 
Bacteria — Phycomycetes  or  alga-like  fungi — Ascomycetes  or 
sac  fungi — Basidiomycetes  or  basidia  fungi — Lichens. 

CHAPTER  XV 

The  Bryophyta 311 

Alternation  of  generations — Multicellular  sexual  organs — Hepati- 
cae  or  liverworts — Musci  or  mosses. 


CONTENTS  XV 

CHAPTER  XVI 

Page 

The  Pteridophyta 325 

The  advance  from  bryophytes  to  pteridophytes — Filicineae  or  ferns 
Lycopodineae  or  club  mosses — Equisetineae  or  horsetails. 

CHAPTER  XVII 

The  Spermatophyta 344 

The  origin  of  the  seed — The  advances  from  pteridophytes  to  seed 
plants — The  flower — Gymnospermae  or  gymnosperms — Angio- 
spermae  or  angiosperms — Dicotyledoneae  or  dicotyledons — 
Monocotyledoneae  or  monocotyledons. 

Index 371 


TO  THE  STUDENT 

Botany  has  sometimes  been  spoken  of  rather  shghtingly  as 
the  ''feminine"  science,  and  indeed  in  the  minds  of  many  people 
it  is  chiefly  associated  with  pleasant  exercises  in  learning  the 
names  of  wild  flowers,  or  with  quaint  individuals  who  wander 
absentmindedly  in  the  woods  and  fields,  trowel  and  tin  box  in 
hand,  and  who  rejoice  in  the  use  of  long  and  unpronounceable 
Latin  words.  That  such  a  conception  of  the  science  of  botany  is 
wholly  inadequate,  a  study  of  the  following  pages  should  amply 
prove.  Plants  are  far  more  than  interesting  playthings.  They 
are  a  conspicuous  and  inescapeable  part  of  the  world  we  inhabit. 
They  make  life  possible  for  us  by  providing  food,  clothing,  fuel, 
shelter,  and  many  other  necessities.  More  important  still,  they 
are  alive  and  thus  endowed  with  those  remarkable  qualities  which 
have  always  made  all  living  things  eagerly  studied  by  man,  not 
only  for  their  own  sake  but  for  the  light  which  they  throw  on 
many  human  problems.  If  education  is  indeed  an  understand- 
ing of  our  surroundings,  surely  no  one  should  pretend  to  be 
educated  well  who  is  unfamiliar  with  plants  and  their  activities. 

A  college  textbook  or  a  college  course  in  any  branch  of  biology, 
however,  ought  to  do  something  more  than  give  familiarity  with 
a  mass  of  facts,  no  matter  how  important  they  may  be.  It 
should  develop  in  us  the  right  attitude  toward  these  facts.  This 
attitude — the  truly  scientific  attitude — is  both  critical  and  inqui- 
sitive. It  should  enable  us  to  find  our  way  to  the  truth  through 
a  maze  of  facts.  This  is  the  attitude  which  distinguishes  a 
truly  educated  man  from  one  who  is  merely  knowledge-cram- 
med, and  a  lack  of  it  is  responsible  for  much  of  the  loose  thinking 
and  false  reasoning  with  regard  to  biological  problems  which  is  in- 
dulged in  by  many  people?. 

Let  us  see  what  this  attitude  demands  and  how  it  may  best  be 
attained.  We  are  all  obliged  to  acquire  a  mass  of  information  if 
we  are  to  live  successfully.  We  must  know  the  character  of  the 
various  things  which  we  eat  and  wear,  the  operation  of  countless 
devices  which  we  use,  the  rules  and  habits  of  other  people,  the 


XVlll  TO  THE  STUDENT 

manifold  concerns  of  our  particular  occupations,  together  with 
countless  other  facts  and  details.  The  world  is  continually- 
asking  us  "whatf",  "where?",  "when?"  and  "how?";  and  we 
are  continually  answering  it  as  best  we  may.  For  most 
people  the  ability  to  reply  successfully  to  these  questions  is 
sufficient,  and  they  desire  nothing  further.  In  a  course  in 
elementary  botany,  such  would  be  content  merely  to  learn  the 
facts  about  plants  so  that  they  might  pass  the  inevitable  examina- 
tion. There  are  always  a  few,  however,  who  are  not  satisfied 
thus  to  take  everything  for  granted,  as  a  lesson  to  be  learned. 
They  want  to  ask  the  world  a  question  in  their  turn,  and  their 
question  is  far  more  profound.  It  is  "why?"  They  crave  not 
merely  knowledge  but  understanding.  They  want  to  penetrate 
the  array  of  facts  to  the  laws  upon  which  these  facts  rest.  To  this 
honorable  band  have  belonged  all  great  thinkers  and  philoso- 
phers, all  discoverers  and  explorers,  all  who  have  helped  to  bring 
mankind  upward  from  thoughtless  savagery  to  rational  civili- 
zation. These  men  have  often  been  ridiculed,  frowned  upon  or 
even  persecuted  for  their  presumptuous  curiosity,  but  they  have 
persisted  through  the  ages  in  that  insatiable  inquisitiveness 
which  has  led  them  to  challenge  the  world,  not  to  accept  it;  to 
penetrate  its  secrets  rather  than  take  all  for  granted.  Their 
spirit  animates  every  true  scientist  and  every  really  educated  man 
today. 

But  the  scientific  attitude  is  not  mere  inquisitiveness.  Brutes 
often  have  plenty  of  that.  Many  people  are  idly  curious  and 
will  accept  any  explanation  offered  them.  The  scientist  must  go 
beyond  this.  Among  the  many  answers  which  come  back  to  him 
when  he  asks  "  why?  "  he  must  be  able  to  discriminate,  to  separate 
the  true  from  the  false.  He  must  weigh  accurately  and  without 
prejudice  the  claims  of  the  various  theories  which  have  been  put 
forward  to  answer  his  questions.  He  must  remain  skeptical  and 
unconvinced  until  adequate  proof  is  at  hand.  He  must  satisfy 
his  reason.     In  the  best  sense  of  the  word,  he  must  be  critical. 

This  attitude  of  critical  curiosity  is  hard  to  gain  and  still 
harder  to  maintain  actively.  It  is  a  useful  practice,  particularly 
when  encountering  a  subject  which  is  new  or  unfamiliar,  to  prod 
ourselves  continually  with  questions  and  problems  which  it 
brings  up,  not  merely  accepting  someone  else's  assertion  but 
trying  things  out  for  ourselves.  The  college  student,  above  all 
men,  should  thus  learn  to  be  his  own  Socrates.     It  is  for  this 


TO  THE  STUDENT  XlX 

purpose  that  the  list  of  "Questions  for  Thought  and  Discussion, " 
which  will  be  found  at  the  end  of  each  chapter  of  the  following 
text,  has  been  gathered.  The  student  can  tr}^  his  hand  at  these, 
and  if  he  has  first  mastered  the  comparatively  few  facts  presented 
in  the  text  he  should  have  no  great  difficulty.  The  answers  to 
some  are  easy,  almost  obvious.  Others  may  be  harder  to  find 
but  will  be  worth  more  in  the  finding.  By  frequent  practice  in 
this  sort  of  exercise  he  will  not  only  develop  and  keep  sharp  his 
curiosity  and  his  ability  to  reason  and  discriminate — the  most 
important  ends  for  him  to  gain  in  such  a  course  as  this — but  he 
will  acquire  almost  unconsciously  a  far  more  thorough  under- 
standing of  the  structures  and  functions  of  plants  than  he  could 
by  merely  attempting  to  memorize  a  list  of  facts.  The  tonic  of 
curiosity  and  the  fresh  air  of  skepticism  are  sovereign  aids  for 
maintaining  our  minds  in  that  state  of  health  and  vigor  in  which 
they  can  steadily  assimilate  a  rich  diet  of  knowledge  without 
becoming  sluggish  and  over-fed. 


BOTANY: 
PRINCIPLES  AND  PROBLEMS 

CHAPTER  I 
THE  SCIENCE  OF  BOTANY 

Botany  may  be  defined  as  that  field  of  precise  and  classified 
knowledge— that  science,  in  other  words— which  deals  with 
plants. 

Together  with  the  members  of  the  animal  kingdom,  plants  are 
distinguished  from  all  other  objects  by  the  fact  that  they  are 
alive.     Botany  and  Zoology,  the  sciences  which  treat  of  these  two 
great  groups  of  living  things,  are  therefore  closely  related  to 
one  another,  dealing  with  many  facts  and  facing  many  problems 
in  common;  and  together  they  constitute  the  broader  scientific 
field  of  Biology,  which  is  concerned  with  the  study  of  Life  in  its 
various  manifestations.     Of  all  branches  of  human  knowledge, 
this  science  of  life  is  perhaps  the  least  understood  and  the  most 
important.     Between  lifeless  things  and  living  things  exists  a 
great  gap  which,  despite  our  advances  in  knowledge,  we  are  as 
yet  unable  to  bridge.     Intricate  machines  and  complex  chemical 
substances  are  familiar  products  of  manufacture,  but  never  have 
we  succeeded  in  constructing  anything  alive  out  of  non-living 
material.     Plants  and  animals  thrive,  multiply  and  die  before 
our  eyes,  but  as  to  many  of  the  processes  which  lie  behind  these 
outward  activities  we  are  still  in  almost  complete  ignorance.     We 
may  describe,  but  often  cannot  explain.     Moreover,  living  things 
are  not  fixed  and  constant  in  their  characteristics  but  have  under- 
gone profound  changes,  from  simpler  beginnings  in  ages  long  past 
to  the  enormous  variety  and  complexity  which  they  exhibit 
today;  but  what  has  been  their  origin,  or  how  and  why  they  have 
progressed  to  their  present  condition,  are  questions  for  the  future 
to  answer.     It  is  this  baffling  and  unexplained  quality  in  life 
which  always  has  keenly  stimulated  the  curiosity  of  man;  and 

1 


2  BOTANY:  PRINCIPLES  AND  PROBLEMS 

since  he  himself  is  a  hving  being,  much  of  his  speculation  and 
philosophy  has  naturally  centered  around  problems  which,  in  a 
broad  sense,  are  biological  in  their  nature.  The  task  of  the 
biologist  in  extending  the  field  of  our  scientific  knowledge  of 
plants  and  animals  therefore  assumes  added  significance  from  its 
relation  to  some  of  the  most  profound  questions  with  which  man- 
kind is  confronted.  As  an  integral  part  of  the  science  of  biology, 
botany  has  already  made  notable  contributions  to  our  knowledge 
of  living  things;  and  so  long  as  man  preserves  that  insatiable 
curiosity  toward  Nature  which  has  distinguished  him  as  a  think- 
ing being,  he  will  always  in  some  measure  be  a  student  of  plants. 

Aside  from  its  theoretical  interest,  botany  is  also  of  ver}^ 
great  practical  importance  to  mankind  because  plants  touch 
human  activities  so  intimately  and  in  so  many  ways.  All  food 
which  nourishes  animals  and  man,  and  makes  life  possible,  comes 
originally  from  a  union  of  water  and  a  simple  gas,  carbon  dioxide, 
in  the  leaves  of  green  plants.  Our  clothing,  our  fuel,  our  drugs 
and  countless  other  necessities  of  civilized  life  are  likewise  con- 
tributed, directly  or  indirectly,  by  members  of  the  plant  kingdom. 
As  a  means  of  quickening  that  intelligent  appreciation  of  his 
surroundings  which  should  distinguish  every  educated  man,  a 
scientific  knowledge  of  plants  is  therefore  of  the  utmost  value. 

The  Subdivisions  of  Botany. — Owing  to  the  great  mass  of 
diverse  facts  which  it  has  accumulated  and  the  many  points  of 
view  from  which  its  problems  have  been  attacked,  every  major 
science  has  necessarily  become  divided  into  sub-sciences,  each  of 
which  makes  its  particular  contribution  to  the  whole.  Thus 
botany  is  composed  of  a  series  of  specialized  sciences.  Three  of 
these — Systematic  Botany,  Plant  Morphology  and  Plant  Physi- 
ology— are  most  worthy  of  note  and  are  themselves  subdivided 
and  recombined  still  further. 

Systematic  Botany  or  Plant  Taxonomy  is  concerned  with  the 
names  of  plants  and  the  classification  of  the  vegetable  kingdom. 
Its  object  is  to  identify  by  name  and  description  all  the  kinds 
of  plants  which  can  be  distinguished,  and  to  arrange  them, 
according  to  their  natural  relationships,  into  those  groups  which 
we  call  species,  genera,  families  and  orders.  Since  these  relation- 
ships can  be  determined  only  after  a  knowledge  of  evolutionary 
history,  the  science  of  Plant  Phytogeny,  which  endeavors  to  trace 
the  genealogy  of  the  plant  kingdom,  is  an  important  adjunct  to 
systematic  botany. 


THE  SCIENCE  OF  BOTANY  3 

Plant  Morphology  is  concerned  with  the  form  and  structure  of 
plants.  Its  object  is  to  describe  the  construction  and  organiza- 
tion of  the  plant  body  and  to  trace  underlying  similarities  in 
form  between  various  plant  groups.  Under  morphology  are 
included  Anatomy,  dealing  with  internal  structure  in  general; 
Histology,  with  the  more  minute  internal  structures;  Cytology, 
with  the  structure  of  the  cell;  Embryology,  with  the  development 
of  the  individual,  and  Experimental  Morphology,  with  the  causes 
which  determine  form  and  structure. 

Plant  Physiology  is  concerned  with  the  functions  of  plants. 
Its  objects  are  to  describe  and  explain  the  various  activities 
by  which  the  life  of  the  plant  is  maintained  and  transmitted  to 
its  offspring.  Physiology  obviously  underlies  the  other  sub- 
divisions of  botany  since  it  touches  the  very  process  of  hving. 
A  branch  of  physiology  particularly  active  today  is  Genetics, 
which  deals  with  the  problems  of  inheritance. 

Aside  from  these  three  major  sub-sciences  there  are  other 
fields  of  botany  which  deserve  mention.  Plant  Ecology  is  con- 
cerned with  the  relations  between  plants  and  the  various  fac- 
tors of  their  environment  such  as  soil,  chmatic  conditions  and 
living  organisms;  and,  in  particular,  with  the  modifications  of 
structure  and  function  which  enable  them  to  react  successfully 
to  changes  in  their  surroundings.  Ecology  necessarily  involves 
both  morphology  and  physiology,  as  well  as  certain  of  the  phys- 
ical sciences.  Plant  Geography  is  concerned  with  the  geographical 
distribution  of  plants,  and  is  intimately  related  both  to  systematic 
botany  and  to  ecology,  as  well  as  to  geology  and  geography. 
Palaeobotany  is  concerned  with  the  structure  and  relationships  of 
fossil  plants,  and  thus  touches  systematic  botany,  morphology, 
and  geology. 

In  addition  to  all  these  aspects  of  botany,  most  of  which  are 
theoretical  rather  than  practical  in  their  bearing,  we  should  not 
fail  to  mention  the  great  group  of  sciences  concerned  with  the 
utilization  and  culture  of  plants.  Economic  Botany,  in  its  nar- 
rower sense,  is  a  study  of  those  plants  which  are  valuable  to 
man  and  of  the  uses  to  which  they  are  put.  The  various  sciences 
commonly  grouped  under  Agriculture  {Soil  Science,  Agronomy, 
Horticulture,  Plant  Pathology,  and  many  others)  together  with 
Forestry,  Pharmacology  and  their  subsidiaries,  are  eminently 
practical,  and  their  close  relationship  to  botany  is  sometimes 
overlooked.     They  are  nevertheless  integral  parts  of  our  science, 


4  BOTANY:  PRINCIPLES  AND  PROBLEMS 

and  for  their  successful  pursuit  demand  a  sound  knowledge  of 
botanical  principles.  Everyone  who  is  concerned  with  plants 
from  a  scientific  point  of  view,  whatever  his  purpose  or  profession, 
is  rightfully  to  be  called  a  botanist. 

The  History  of  Botany.     The  Classical  PenW.— Botany  as 
we  know  it  today  is  the  result  of  a  long  term  of  observation 


® 

f^^tt^S'^ 

"^^^^^0M^p 

■^''^^^^^^fl 

Fig.   1.— Aristotle,  384-322  B.C. 


and  inquir3^  As  do  so  many  other  sciences,  it  looks  back  to 
the  fertile  speculations  of  the  Greeks  for  the  first  definite  expres- 
sion of  its  problems  and  principles.  The  "nature"  of  plants 
was  studied  by  Aristotle  (384-322  B.C.,  Fig.  1)  who  saw 
clearly  certain  of  the  broader  problems  of  plant  and  animal  life, 
and  whose  sagacious  comments  thereon  are  still  worthy  of  our 
attention.  It  is  his  disciple  Theophrastus  of  Eresus  (371-287 
B.C.),  however,  whom  botanists  have  generally  regarded  as 
the  father  of  their  science.  This  keen  naturalist  accumulated 
a  great  mass  of  information  with  regard  to  plants  and  discussed 
their  various  characteristics.  Rome  also  had  her  share  in  the 
development  of  plant  science,  notably  through  the  contributions 
of  Pliny   the   Elder  (23-79  A.D.),  whose   "Natural   History," 


THE  SCIENCE  OF  BOTANY  5 

a  compendium  of  facts  and  fancies  about  living  things,  was 
long  a  storehouse  of  botanical  information.  Dioscorides,  living 
at  about  the  time  of  Nero,  studied  plants  for  their  medicinal 
properties  and  holds  an  important  place  historically  in  both 
botany  and  medicine. 

The  Herbalists. — After  this,  its  classical  period,  botany  went 
into  that  profound  eclipse  suffered  by  all  sciences  during  the 
Middle  Ages.  The  teachings  of  the  ancient  masters  were  jealously 
preserved  and  were  commented  upon  and  dissected,  but  the 
thought  of  extending  knowledge  by  direct  observation  and 
experiment  was  held  to  be  almost  sacrilegious.  About  the  begin- 
ning of  the  sixteenth  century,  however,  a  group  of  open-minded 
men  living  in  the  Rhine  valley  and  its  adjacent  regions  under- 
took to  explore  the  plant  kingdom  afresh  for  themselves.  They 
were  interested  in  plants  chiefly  for  the  curative  virtues  to  be 
found  therein,  and,  paying  scant  attention  to  the  doctrines  and 
dogmas  of  the  ancients,  they  went  about  describing  and  drawing 
with  fidelity  the  various  kinds  of  plants  which  flourished  in  their 
native  countries.  From  the  numerous  herb-books  or  "Herbals" 
in  which  the  resulting  discoveries  were  published,  these  pioneers 
have  been  known  as  the  "Herbalists".  They  endeavored  to 
distinguish  clearly  the  different  species  from  one  another,  and 
proposed  certain  crude  methods  of  classifying  the  plant  kingdom. 
So  unprejudiced  and  free  from  the  conventional  dogmatism  of 
the  age  was  their  whole  attitude  that  the  Herbalists  are  generally 
regarded  as  the  fathers  of  modern  botany. 

The  Modern  Period.— The  first  extensive  and  thorough  classi- 
fication of  plants  was  that  proposed  in  1583  b}^  the  Italian 
botanist  Cesalpino  (1519-1603).  Combining  an  acquaintance 
with  the  ancients  and  an  intimate  first-hand  knowledge  of  plants, 
he  laid  down  certain  principles  which  were  the  basis  of  systematic 
botany  for  many  years.  Modern  taxonomy,  however,  dates 
from  the  publication  of  the  "Species  Plantarum"  by  the  great 
Swedish  naturahst  Linnaeus  (1707-1778,  Fig.  2),  in  1753.  In 
this  monumental  book  all  the  plant  species  known  at  that  time 
were  named,  carefull}^  described,  and  arranged  according  to  a 
definite  sj^stcm. 

Although  the  earl}"  work  in  botany  was  thus  concerned  chiefly 
with  taxonomy,  the  study  of  plant  structures  was  not  neglected 
Grew  (1628-1711)  in  England,  and  Malpighi  (1628-1694)  in 
Italy,  were  keenly  interested  in  the  internal  construction  of  the 


6  BOTANY:  PRINCIPLES  AND  PROBLEMS 

plant  bod}^,  and  their  works  on  "phytotomy  "  laid  the  foundation 
for  our  modern  knowledge  of  plant  morphology.  The  continued 
improvement  in  the  compound  microscope  made  possible  more 
complete  and  accurate  knowledge  of  the  way  plants  are  con- 
structed, and  led  to  the  formulation  bj^  Schleiden,  in  1838,  of  the 
Cell  Theory,  which  states  that  the  cell  is  the  unit  of  structure  in 
plants  and  that  protoplasm  is  its  essential  constituent.  From 
this  beginning,  modern  anatomy  and  cytology  have  added  a  great 


Fig.  2.— Carolus  Linnaeus  (Carl  von  Linne),  1707-1778. 

body  of  facts  to  our  knowledge  of  the  structure,  growth,  and 
reproduction  of  the  plant  body. 

The  ancients  and  the  early  modern  botanists  for  the  most 
part  had  fanciful  and  inaccurate  ideas  as  to  the  way  in  which  the 
plant  carried  on  its  various  functions,  an  ignorance  largely  due 
to  the  undeveloped  state  of  the  sciences  of  physics  and  chemistry 
at  the  time.  It  was  not  until  the  latter  part  of  the  eighteenth 
century  that  modern  plant  physiology  became  definitely  estab- 
Hshed.  Oxygen  was  discovered  by  Priestley  in  1774,  and  five 
years  later  a  Dutch  physician,  Ingenhousz,  observed  that  green 
plants  in  the  light  take  in  carbon  dioxide  and  give  off  oxygen, 
and  that  all  plants  give  off  a  certain  amount  of  carbon  dioxide. 
These  gas  exchanges  were  accurately  measured  by  de  Saussure 
in  the  early  years  of  the  nineteenth  century,  and  some  of  the 


THE  SCIENCE  OF  BOTANY  7 

important  facts  of  plant  physiology  thus  became  established. 
Since  that  time  the  development  of  modern  chemistry  and 
physics  has  made  possible  a  steady  advance  in  our  knowledge  of 
the  physiological  processes  of  living  things. 

The  publication  of  the  "Origin  of  Species"  by  Charles  Darwin 
(Fig.  3)  in  1859  resulted  in  a  general  acceptance  among  scientists 
of  the  theory  of  evolution.     A  recognition  of  the  fact  that  the 


Fig.  3.— Charles  Darwin,  1809-1882. 

plants  of  today  have  been  slowly  developed  from  simpler  ancestors 
has  had  a  profound  effect  upon  botanical  science  and  has  stimu- 
lated a  great  interest  in  reconstructing  the  "family  tree"  of  the 
plant  kingdom  and  establishing  a  really  "natural"  system  of 
classification,  based  on  actual  relationship,  to  replace  the  artificial 
systems  of  Linnaeus  and  his  predecessors.  It  has  led  also  to  a 
more  intensive  study  of  the  laws  of  variation  and  inheritance,  and 
the  causes  and  method  of  evolution.  This  has  been  encouraged 
still  further  by  the  discovery  of  Mendel's  Law  of  Inheritance, 
propounded  in  1866,  disregarded  for  many  years,  and  finally 
brought  to  the  attention  of  biologists  again  in  1900. 

The  present  state  of  the  science  of  botany,  then,  is  the  result 
of  a  long  period  of  slow  development,  in  which  truth  has  been 
gradually  separated  from  error  and  our  present  vast  store  of  facts 
amassed.  With  each  advance,  new  questions  have  arisen  and 
new  fields  of  investigation  have  opened,  until  the  science  has 


8  BOTANY:  PRINCIPLES  AND  PROBLEMS 

broadened  from  a  mere  discussion  of  the  names  and  properties 
of  medicinal  herbs  to  an  attack  upon  the  fundamental  problems 
of  life  itself. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

1.  Name  three  important  differences  between  a  typical  plant  and 
a  typical  animal. 

2.  Which  do  you  think  is  the  most  important  of  all  the  various 
contributions  made  by  plants  to  man's  welfare? 

3.  What  great  industries  are  founded  primarily  on  plants? 

4.  Could  man  get  along  better  without  his  domestic  animals  or 
without  his  cultivated  plants?     Explain. 

5.  What  is  the  difference  between  a  botanist  and  a  person  who  is 
merely  interested  in  plants? 

6.  Would  you  consider  the  following  to  be  botanists: 

A  farmer?  A  cabinet  maker? 

A  florist?  A  forester? 

A  sanitary  engineer?  A  landscape  architect? 

7.  If  a  person  makes  a  careful  study  of  ail  the  species  and  varieties 
of  wheat,  classifying  them  and  finding  their  correct  names,  in  what  field 
of  botany  is  he  at  work? 

8.  If  he  studies  the  structure  of  the  wheat  stem,  in  what  field  of 
botany  is  he  at  work? 

9.  If  he  studies  the  manner  in  which  food  is  manufactured  by  the 
wheat  plant,  in  what  field  of  botany  is  he  at  work? 

10.  If  he  studies  the  ways  in  which  wheat  plants  respond  to  changes 
in  their  environment,  in  what  field  of  botany  is  he  at  work? 

11.  If  he  studies  inheritance  in  wheat,  in  what  field  of  botany  is  he  at 
work  ? 

12.  If  he  studies  the  geogi'aphical  distribution  of  wheat,  in  wliat 
field  of  botany  is  he  at  work? 

13.  If  he  studies  the  various  uses  to  which  wheat  may  be  put,  in  what 
field  of  botany  is  he  at  work? 

14.  In  what  ways  may  a  knowledge  of  botany  be  of  value  in  everyday 
life? 

15.  Name  a  definite  and  specific  problem,  of  practical,  "dollars- 
and-cents"  importance,  in  agriculture  or  any  other  Hne  of  industry,  for 


THE  SCIENCE  OF  BOTANY  9 

the   snlviiifj;   of   which    a  knowledge   of   systematic   botany    would    be 
valuable. 

16,  Name  such  a  pmblem  in  which  a  knowknlgi!  of  plant  morphology 
would  be  valuable. 

17,  Name  such  a  problem  in  which  a  knowledge  (jf  plant  physiology 
would  be  valuable. 

18,  Name  such  a  problem  in  which  a  knowledge  of  plant  genetics 
would  be  valuable. 

19,  Name  such  a  problem  in  which  a  knowledge  of  plant  ecology 
would  be  valuable. 

20,  Give  an  instance  of  a  practical  situation  in  which  a  man  with  a 
knowledge  of  both  the  scientific  and  the  practical  sides  of  agriculture 
would  have  an  advantage  over  a  man  who  knew  only  the  practical  side. 
Why  would  he  have  this  advantage? 

21,  How  is  a  knowledge  of  chemistry  important  to  a  botanist? 

22,  How  is  a  knowledge  of  physics  important  to  a  botanist? 

23,  How  is  a  knowledge  of  meteorology  important  to  a  botanist? 

24,  How  is  a  knowledge  of  geology  important  to  a  botanist  ? 

25,  Why  was  systematic  botany  the  first  aspect  of  botany  to  be 
studied  scientifically? 

26,  Plants  have  always  been  used  far  more  for  food  than  for  medicine, 
but  despite  this  fact,  the  science  of  botany  in  its  early  days  was  almost 
entirely  in  the  hands  of  men  interested  in  medicine  rather  than  in  agri- 
culture or  any  other  branch  of  practical  science.  How  do  you  explain 
this? 

27,  Why  was  a  knowledge  of  the  minute  structure  of  plants  impossible 
for  the  ancients  to  acquire? 

REFERENCE  PROBLEMS 

1.  \Miat  is  meant  by  a  science? 

2.  State  briefly  the  doctrine  of  Spontaneous  Generation  and  c.\i)lain  why 
it  is  no  longer  accepted  by  scientific  men. 

3.  The  following  "applied"  sciences  are  all  closely  related  to  botany  and 
are  founded  upon  it.     With  what  does  each  deal? 

Horticulture  Forestry 

Agronomy  Pharmacology 


10  BOTANY:  PRINCIPLES  AND  PROBLEMS 

4.  Summarize  the  life  and  work  of  Aristotle  and  state  his  important 
contributions  to  botany. 

6.  Summarize  the  life  and  work  of  Linnaeus  and  state  his  important  con- 
tributions to  botany. 

6.  Summarize  the  life  and  work  of  Darwin  and  state  his  important  con- 
tributions to  botany. 

7.  What  change  in  geological  theory  occurred  at  about  the  same  time 
that  the  theory  of  evolution  won  wide  acceptance  (in  the  latter  half  of  the 
nineteenth  century)? 

8.  Give  the  derivation  of  each  of  the  following  terms  and  explain  in  what 
way  it  is  an  appropriate  one: 

Taxonomy  Physiology  Ecology 

Morphology  Phylogeny  Genetics 


CHAPTER  II 
INTRODUCTORY  SURVEY 

Before  commencing  an  intensive  study  of  any  aspect  of  botani- 
cal science  or  of  any  particular  problem  which  deals  with  plants, 
it  will  be  well  for  us  to  make  a  brief  survey  of  the  plant  kingdom 
as  a  whole,  and  of  some  of  the  more  important  structures  and 
functions  of  plants  in  general. 

The  Plant  Kingdom. — About  250,000  different  kinds  or  species 
of  plants  have  been  discovered  and  described,  and  every  year 
botanical  exploration  and  careful  study  bring  more  of  them  to  our 
knowledge.  We  have  seen  that  the  problem  of  systematic  botany 
is  to  name  this  host  of  plants  and  to  arrange. and  classify  its 
members  in  a  logical  system.  Over  many  of  the  details  of  such 
a  classification  difference  of  opinion  still  exists,  but  there  is  now 
rather  general  agreement  as  to  the  main  groups  into  which  the 
plant  kingdom  should  be  divided.  Four  such  divisions  are 
commonly  recognized : 

A.  The  Thallophytes. — These  are  lowl}-  plants,  various  in  their 
structure,  activities,  and  methods  of  reproduction,  but  agreeing 
in  the  possession  of  a  simple  body  without  roots  or  leaves  and  in 
multiplying  by  single-celled  spores.  The  majority  of  Thallo- 
phytes inhabit  water  or  moist  places  and  are  small  and  soft-bodied 
plants. 

There  are  two  main  series  of  Thallophytes:  The  Algae  (Fig.  4), 
which  possess  the  green  pigment  cJdorophyll  and  are  thereby  able 
to  manufacture  their  own  food,  and  which  include  all  the  seaweeds 
and  their  fresh-water  allies;  and  the  Fungi  (Fig.  5),  which  lack 
chlorophyll  and  consequently  are  obliged  to  obtain  their  food 
from  hving  animals  and  plants  or  from  dead  organic  material. 
Here  belongs  the  vast  array  of  bacteria,  molds,  blights,  rusts, 
toadstools,  mushrooms,  and  similar  plants,  many  of  which  live  as 
parasites  and  are  often  the  cause  of  serious  diseases  of  man  and 
the  lower  organisms. 

B.  The  Bnjophyics  or  Moss  Plants. — These  plants  are  dis- 
tinguished from  the  Thallophytes  chiefly  by  their  more  highly 

11 


12 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


developed  w>xual  structures  and  their  more  complicated  methods 
of  reproduction.  The  plant  body  of  the  Bryophytes  has  no  roots, 
and  in  many  cases  consists  of  only  a  flat,  strap-like  mass  of  green 
tissue,  but  the  higher  members  of  the  group  possess  very  simple 
stems  and  leaves.  The  plants  are  small  and  inconspicuous, 
and  generally  thrive  best  in  moist  situations.  Bryophytes  are 
subdivided  into  the  simple  and  lowly  Liverworts  (Hepaticae)  and 
the  commoner  and  more  highly  specialized  Mosses  (Musci,  Fig.  6). 


Fig.   4. — An  Alga.      One  of  the  rock-weeds  {Fucus  spiralis)  growing  on  a  rock 
between  tide-marks.      {Photo  by  M.  A.  Howe). 

C.  The  Pteridophytes  or  Fern  Plants.— These  possess  true 
roots,  stems,  and  leaves,  essentially  similar  in  structure  to  those 
of  the  Seed  Plants,  but  they  still  reproduce  by  spores  rather  than 
by  seeds.  Compared  with  Bryophytes,  the  plant  body  is  large 
and  vigorous,  and  it  is  well  adapted  to  life  on  land.  The  three 
important  subdivisions  of  the  Pteridophytes  are:  The  Ferns 
(Filicales,  Fig.  7),  possessing  large  and  feathery  leaves  on  the 
backs  of  which  the  spores  are  produced;  the  Club  Mosses  or  Ground 
Pines  (Lycopodiales),  which  have  spore-bearing  cones,  solid 
stems  and  scale-like,  spirally  arranged  leaves;  and  the  Horsetails 
(Equisetales)  also  possessing  cones  but  with  jointed,  hollow  stems 
and  minute,  whorled  leaves. 


INTRODUCTORY  SURVEY  13 

D.  The  Spermatophytes  or  Seed  Plants. — The  dominant  and 
familiar  portion  of  the  earth's  vegetation  today  consists  of  these 
plants,  which  are  well  adapted  for  life  on  land  and  often  attain 


Fig.  5. — A  Fungus.      One  of  the  gill  fungi,  Pleurotus,  growing  on  a  log. 


great  size.  Their  distinctive  feature  is  the  production  of  a 
complex,  many-celled  reproductive  body,  the  seed,  in  which  is 
contained  an  embryo  plant  and  a  supply  of  stored  food. 

Seed  Plants  are  very  numerous  and  exceedingly  varied  in  form 
and  structure,  ranging  from  small  and  delicate  herbs  to  huge  trees 
over  three  hundred  feet  tall.  They  are  the  most  conspicuous  and 
best  known  of  all  the  divisions  of  the  plant  kingdom,  and  provide 
the  great  bulk  of  the  foods,  timbers,  fibers,  and  other  vegetable 
products  which  form  the  basis  of  our  civiUzation. 


14  BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fig.  6. — A  Moss.  "A  hair-cap  moss,  Polytrichum  juniperinum,  with  ripe 
capsules.  {From  A.  J.  Grout,  "Mosses  with  Lens  and  Camera."  Copyright  by 
the  author). 


of  the  shield-ferns  (Aspidiumspinuloium). 


INTRODUCTORY  SURVEY 


15 


Two  major  subdivisions  of  the  Seed  Plants  are  recognized: 
The  Gymnosperms  (Fig.  8),  which  have  primitive,  often  cone- 
Hke  flowers  and  bear  their  seeds  openly  exposed  on  scales  as 


r  tree  (Abicf!  >wbilis). 


HI  our  common  coniferous  trees;  and  the  Angiospenns  (Fig.  9), 
in  which  there  is  usually  a  typical  flower  with  its  various  floral 
parts,  including  an  ovary  in  which  the  seeds  are  enclosed  during 
their  development.  There  are  only  about  450  species  of  Gymno- 
sperms living  today,   but  the  Angiosperms  are  an  enormous 


16 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


group  of  more  than  130,000  species  and  are  our  most  familiar 
plants.     They   are   divided   again   into   two   main   groups,   the 


lii 

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i. 

[p!?®Si.£i^vi 

s 

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m^ 

r  2 

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^  ^u, ■'■■■'■ ''^ 

|.;, 

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.-'l.r^,. 

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^^^^^^^^K^^ 

"^H 

^^^^K' 

i^ 

_,       M 

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I'lG.  9. — A  Seed  Plant  (Angiosperm).     Hawthorn  tree  (Crataegus). 


Dicotyledons  and  the  Monocotyledons,   which  differ  from  each 
other  in  the  structure  of  the  seed,  leaf,  stem,  and  flower. 


INTRODUCTORY  SURVEY  17 

Underlying  the  differences  b}-  which  these  various  groups  are 
distinguished  from  one  another,  there  are  many  fundamental 
similarities  in  structure  and  function  which  are  common  to  all 
plants;  but  the  marked  changes  which  appear  as  we  pass  from 
the  lowest  to  the  highest  types  make  very  difficult  a  concise 
description  of  the  characteristics  of  the  plant  kingdom  as  a  whole. 
It  will  therefore,  be  profitable  for  us  to  confine  our  attention, 
at  first,  mainly  to  those  plants  which  are  most  familiar  to  every- 
one and  of  most  obvious  and  immediate  interest  to  man — the 
Seed  Plants.  The  sciences  of  morphology  and  physiology, 
as  exemplified  by  the  Seed  Plants,  will  accordingly  be  the  chief 
object  of  study  in  the  first  portion  of  this  book.  In  these  early 
chapters  we  should  be  careful  to  remember  that  only  one  division 
of  the  plant  world — albeit  the  most  important  one — is  being 
considered,  although  many  of  the  facts  and  principles  there  estab- 
lished are,  of  course,  valid  for  all  plants.  In  the  last  chapters  of 
the  text  we  shall  discuss  in  some  detail  the  lower  members  of  the 
vegetable  kingdom,  and  the  respects  in  which  they  differ  from 
each  other  and  from  the  Seed  Plants. 

The  great  variety  of  plant  types  and  the  diversity  of  condi- 
tions under  which  they  live  renders  it  difficult  to  make  general 
statements  about  plants  which  are  universally  true,  for  exceptions 
to  any  such  statement  may  usually  be  found.  Indeed,  varia- 
bility is  one  of  the  most  notable  characteristics  of  all  life.  The 
student  should  therefore  bear  in  mind  that  man}-  of  the  facts 
and  principles  set  forth  briefly  and  simply  in  an  clementar}-  text 
are  to  be  taken  as  true  for  typical  cases  and  under  ordinary  con- 
ditions, and  he  should  be  careful  not  to  accept  them  as  necessarily 
and  universally  true  for  all  plants  and  under  all  conditions. 
Living  things  are  too  complicated  to  be  described  completely 
in  simple  formulas. 

The  Structures  and  Functions  of  Plants. — A  notable  character- 
istic of  plants,  which  they  share  with  all  other  living  things,  is 
their  very  definite  bodily  form.  This  form  is  not  merely  external 
or  accidental  but  is  the  mark  of  a  fundamental  organization  or 
"division  of  labor"  within  the  plant  itself.  The  individual  is 
made  up  of  a  series  of  distinct  and  visibly  different  parts,  each 
of  which  possesses  a  specific  shape  and  performs  a  specific 
function  (Fig.  10).  These  parts  are  called  organs.  The  root, 
the  stem,  the  leaf,  the  flower,  the  fruit,  and  the  seed,  as  well  as 

2 


18  BOTANY:  PRINCIPLES  AND  PROBLEMS 


Flower 


Roof    »-- 


>■  Reproduction 


;    Photosynthesis 
^    Transpiration 

I    Respiration  (In  Part) 


^'-'   Anchorage 
Absorpti 


Fig.   10. — The  important  structures  and  functions  of  a  seed  plant,  illustrated 
diagramma'tically. 


INTRODUCTORY  SURVEY  19 

the  various  subordinate  parts  of  which  each  may  be  composed, 
such  as  bud,  petiole  or  stamen,  are  some  of  these  organs. 

The  organ,  in  turn,  is  not  homogeneous  in  texture  and  con- 
struction but  is  made  up  of  a  group  of  tissues,  each  of  which  per- 
forms a  particular  task  contributory  to  the  general  function  of 
the  entire  organ.  Thus  a  stem  may  be  composed  of  bark  tissue, 
cortical  tissue,  bast  tissue,  cambial  tissue,  woody  tissue,  and  pith 
tissue,  each  of  them  playing  some  role  in  the  economy  of  the 
stem  as  a  whole.  One  type  of  tissue  may,  of  course,  enter  into 
the  construction  of  several  organs;  for  there  is  woody  tissue, 
for  example,  in  the  root,  the  stem,  and  the  leaf. 

A  tissue,  in  turn,  is  an  aggregation  of  cells,  those  ultimate 
units  of  structure  and  function  in  all  organisms.  Each  plant 
cell  is  a  minute  but  distinct  bit  of  living  substance,  or  protoplasm, 
with  nucleus,  plastids,  sap-cavity,  and  other  structures  of  its 
own,  and  is  usually  enclosed  in  a  cellulose  wall.  The  individual 
plant  ma}^,  indeed,  be  regarded  as  a  huge  colony  of  minor  indi- 
viduals, the  cells,  each  performing  some  particular  function  in 
the  whole  and  all  bound  together  to  their  mutual  advantage 
A  knowledge  of  the  structure  and  activities  of  cells  is  the  founda- 
tion upon  which  our  understanding  of  plants  and  animals  must  be 
built. 

The  Root  and  Its  Functions. — The  root  is  that  organ  which 
anchors  the  plant  in  the  soil  and  which  absorbs  therefrom  water 
and  simple  inorganic  nutrient  materials.  Aside  from  these 
primary  functions,  it  frequently  serves  as  a  storehouse  for  reserve 
food  and  often  assumes  other  secondary  duties.  The  root 
system  may  consist  of  a  single  strong  tap-root,  with  weak  lateral 
branches,  or  of  a  much-branched  series  of  smaller  fibrous  roots. 
The  function  of  absorption  takes  place  in  minute  root-hairs, 
delicate  outgrowths  from  the  surface  cells  just  behind  the  young 
and  growing  root-tip.  Into  these  root-hairs,  by  the  process  of 
os77iosis,  pass  water  and  dissolved  substances  (chiefly  nutrient 
mineral  salts)  from  the  soil. 

The  Leaf  and  Its  Functions. — The  typical  leaf  is  a  broad  and 
thin  structure,  green  in  color  and  freely  exposed  to  air  and  light. 
Its  essential  portion,  the  blade,  is  usually  supported  by  a  stalk, 
the  petiole.  From  the  leaf  tissues,  the  water  which  has  entered 
the  root  and  ascended  the  stem  is  evaporated  in  the  process  of 
transpiration.  Most  of  the  internal  cells  of  the  leaf  possess  a 
green  pigment,  chlorophijll,  which  can  utilize  the  energy  of  hglit 


20  BOTANY:  PRINCIPLES  AND  PROBLEMS 

to  combine  carbon  dioxide  (coming  from  the  air)  and  water  (com- 
ing from  the  soil)  into  grape  sugar,  a  simple  carbohydrate  food. 
This  process  of  photosynthesis  is  entirely  confined  to  green  plants. 
By  it  is  manufactured  all  the  food  which  sustains  the  lives  of 
plants  and  animals  and  of  man  himself,  for  all  the  complex  foods 
with  which  we  are  familiar  have  been  built  up  by  progressive 
modifications  of  grape  sugar  produced  in  green  leaves. 

The  Stem  and  Its  Functions. — The  stem  normally  holds  aloft 
in  the  air  the  leaves  and  reproductive  organs,  and  serves  as  a 
highway  for  transportation  of  water  and  nutrient  materials  from 
the  root  to  the  leaf,  and  of  manufactured  food  from  the  leaf  to 
other  parts  of  the  plant  body.  Stems  may  be  comparatively 
small  and  soft,  as  in  herbaceous  plants,  or  stout  and  woody,  as  in 
trees  and  shrubs.  They  are  occasionally  modified  for  special 
functions,  such  as  food-storage  or  photosynthesis. 

The  Reproductive  Organs  and  Their  Function. — The  sole 
function  of  these  organs  is  the  production  of  offspring,  through 
which  the  life  of  the  plant  may  be  transmitted  to  succeeding 
generations.  The  flower,  typically  composed  of  sepals,  petals, 
stamens,  and  pistil,  is  concerned  with  effecting  fertilization,  or 
the  union  of  male  and  female  sexual  cells;  the  fruit  protects  the 
growing  seeds  and  often  aids  in  their  dispersal,  and  the  seeds  are 
young  plants  themselves  in  embryo,  protected  by  a  coat,  provided 
with  a  supply  of  food,  and  ready  to  begin  their  independent 
growth  and  development  whenever  favorable   conditions  appear. 

Metabolic  Processes. — Certain  physiological  activities  of  the 
plant  are  not  confined  to  any  one  organ  but  are  characteristic 
of  living  substance  or  protoplasm  wherever  it  may  be.  Notable 
among  these  are  digestion,  whereby  food  is  rendered  soluble; 
assimilation,  whereby  such  digested  food  is  incorporated  into 
protoplasm,  and  respiration,  whereby  the  supply  of  energy 
necessary  for  the  plant's  activities  is  released  through  the  break- 
ing down  of  living  tissue,  with  the  consequent  absorption  of 
oxygen  and  liberation  of  carbon  dioxide. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

28.  There  are  almost  as  many  species  of  Thallophytes  as  of  Seed 
Plants,  but  the  latter  are  much  more  familiar  to  most  people  than  are 
the  former.     Why? 

29.  Which  of  the  four  main  divisions  of  the  plant  kingdom  do  you 
believe  is  the  oldest?    Why? 


INTRODUCTORY  SURVEY  21 

30.  Which  of  these  four  divisions  contains  the  most  plants  useful  for 
food? 

31.  In  which  of  these  four  divisions  are  the  largest  plants  found? 
In  which  the  smallest? 

32.  To  which  of  these  four  divisions  do  plants  growing  in  the  ocean 
chiefly  belong?     To  which  do  land  plants  chiefly  belong? 

33.  Name  some  Thallophytes  which  are  useful  to  man  and  some  which 
are  harmful.  Name  some  Spermatophytes  which  are  useful  and  some 
which  are  harmful. 

34.  Which  of  the  four  divisions  contains  the  largest  number  of  plant 
species  which  are  harmful  to  man  and  to  his  domestic  animals  and 
plants?     Which  contains  the  largest  number  of  useful  species? 

35.  Bryophytes  and  Pteridophytes  have  much  fewer  species  than 
either  Thallophytes  or  Spermatophytes.  What  reason  can  you  suggest 
for  this  fact? 

36.  Under  what  climatic  conditions  are  Pteridophytes  most  conspicu- 
ous and  abundant? 

37.  What  are  the  advantages  of  organization  (the  "division  of  labor") 
in  a  plant? 

38.  Which  of  the  four  main  divisions  of  plants  do  you  think  shows 
the  highest  degree  of  organization  within  its  plant  body? 

39.  In  general,  do  you  think  that  it  has  been  the  most  highly  organized 
or  the  less  highly  organized  plants  which  have  been  most  successful? 
Explain  your  answer. 

40.  Why  are  living  things  called  "organisms?" 

41.  Are  the  most  important  agricultural  crops  derived  from  the 
root,  the  stem,  the  leaf,  or  the  fruit?     Explain. 

42.  Roots  are  confined  almost  entirely  to  land  plants.     Explain. 

43.  Give  an  example  of  a  root  which  serves  as  a  store-house  for  a 
large  amount  of  food. 

44.  What  advantage  has  the  root  over  the  stem  as  a  place  for  food 
storage? 

45.  Name  ten  cultivated  plants  in  which  the  root  is  the  organ  useful 
to  man. 

46.  Name  a  few  plants  in  which  the  stem  is  very  much  reducetl. 
Under  what  important  disadvantage  are  such  plants? 


22  BOTANY:  PRINCIPLES  AND  PROBLEMS 

47.  Name  ten  cultivated  plants  in  which  the  stem  is  the  organ  useful 
to  man. 

48.  The  blade  of  an  ordinary  leaf  is  broad  and  thin.  Explain  the 
advantage  of  this  to  the  plant. 

49.  Name  ten  cultivated  plants  in  which  the  leaf  is  the  organ  useful 
to  man. 

50.  What  makes  reproduction  necessary  among  plants  and  animals? 

51.  Name  ten  cultivated  plants  in  which  the  fruit  or  seed  is  the  organ 
useful  to  man. 

52.  What  important  resemblances  are  there  between  the  physiology 
of  a  typical  plant  and  of  a  typical  animal?  What  important  physio- 
logical differences  are  there  between  these  two  groups  of  organisms? 

REFERENCE  PROBLEMS 

9.  Are  there  more  species  of  plants  or  of  animals  on  the  earth  today? 

10.  Are  there  any  plants  which  lack  roots?  which  lack  leaves?  which  lack 
a  stem?  which  lack  reproductive  organs? 

11.  Give  the  derivation  of  each  of  the  following  terms  and  explain  in  wliat 
way  it  is  an  appropriate  one: 

Thallophyte  Bryophyte  Pteridophyte 

Spermatophyte 


CHAPTER  III 
THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS 

It  is  impossible  to  study  the  plant  as  a  living  organism  without 
an  understanding  of  the  surroundings  or  environment  in  which  it 
grows.  For  most  of  the  Seed  Plants,  the  physical  environ- 
ment may  be  divided  roughly  into  the  soil  and  the  atmosphere. 
Of  these  two,  the  soil  is  much  the  more  complex — physically, 
chemically,  and  biologically.  This  fact,  together  with  the 
profound  effect  produced  upon  the  plant  by  changes  within  it, 


Fig.    11. — Composition   of   soil.     Graph   showing   the   percentage    composition, 
by  volume,  of  a  rich,  loam  soil. 

warrants  us  in  devoting  to  the  soil  a  brief  preliminary  discussion 
before  we  consider  in  detail  the  structures  and  functions  of  the 
plant  itself. 

The  soil  has  three  main  uses  in  the  plant's  economy:  It 
provides  an  anchorage  and  support  whereby  the  plant  may  be 
held  firmly  in  position ;  it  furnishes  the  supply  of  water  which  the 
plant  uses,  and  it  contributes  certain  mineral  salts  essential  to  the 
plant's  successful  activity. 

23 


24  BOTANY:  PRINCIPLES  AND  PROBLEMS 

Soils  vary  much  in  physical  texture,  chemical  composition, 
depth,  origin,  richness,  and  other  respects,  but  all  are  normall}^ 
made  up  of  a  mixture  of  distinct  components,  each  of  which  has 
its  particular  influence  upon  the  life  of  plants.  These  com- 
ponents are  rock-particles,  water,  air,  humus,  dissolved  sub- 
stances, and  organisms  (Fig.  11). 

Rock  Particles. — The  bulk  and  the  basic  material  of  a  soil 
is  composed  of  small,  angular  particles  which  have  been  formed 
by  disintegration  of  rock.  These  make  up  90  per  cent  of  the 
weight  of  ordinary  good  soil,  furnish  the  necessary  anchorage  for 
the  plant,  and,  through  the  substances  dissolved  from  their 
surfaces,  contribute  to  the  supply  of  available 
nutrient  materials.  The  particles  vary  greatly 
in  size,  from  those  of  fine  clay  to  those  of  coarse 
gravel.  They  also  differ  in  shape  and  in  chemical 
composition  according  to  the  type  of  rock  pro- 
ducing them.  The  irregularity  of  contour  which 
iiG.    12.— Soil-  these  particles  display  makes  it  impossible  for 

crumbs  or  floccules.  ^  r-     ^  v 

Much  enlarged.  them  to  fit  very  closely  together,  and  a  consider- 
able amount  of  space  (pore-space)  is  thus  left 
between  them  which  may  be  occupied  by  air  or  by  water.  In 
soils  which  are  in  good  condition  for  the  growth  of  ordinary 
plants  the  particles  cohere  in  groups  to  form  crumbs  or  floccules 
(Fig.  12),  the  component  grains  of  which  are  held  together  by 
water-films  or  by  such  a  cementing  substance  as  clay.  One 
important  purpose  of  tillage  is  to  impart  this  crumb  structure  or 
flocculation  to  a  soil.  At  the  soil  surface,  by  the  direct  action  of 
the  rain  or  by  other  means,  these  crumbs  may  be  broken  into 
their  constituent  particles,  which  then  pack  closely  together 
and  on  drying  harden  into  a  firm,  clay-like  crust. 

Water. — Water  is  of  vital  importance  to  plants  in  many  ways. 
It  constitutes  the  great  bulk  of  their  bodily  material;  it  enters 
into  the  manufacture  of  food;  it  assists  in  maintaining  the  plump- 
ness and  rigidity  of  the  tissues;  and,  in  its  capacity  as  a  solvent, 
it  serves  as  the  general  medium  in  which  most  physiological 
processes  are  carried  on. 

The  chief  source  of  soil  water,  and  in  most  cases  the  only  one, 
is  the  rain  which  falls  upon  the  soil  surface.  Various  fates 
await  this  water  (Fig.  13).  A  considerable  part  of  it  may  not 
enter  the  soil  at  all  if  the  surface  is  hard  or  the  rainfall  heavy, 
but  may  drain  away  instead.     This  run-off  is  lost  to  plants,  and 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS 


25 


may  even  do  much  harm  by  washing  away  a  portion  of  the  soil 
itself.  The  water  which  does  enter  the  soil  may  either  percolate 
downward  between  the  j^articles  under  the  influence  of  gravity, 
or  may  be  hold  in  the  soil  by  capillarity. 

Percolating  or  gravitational  water  passes  downward  rapidly 
if  the  soil  particles  arc  coarse,  more  slowly  if  they  are  finer,  until 
it  arrives  at  a  level  where  all  the  soil  spaces  are  filled  with  standing 

/^;?  /    Transpiration 


Run- off 


Zone  of 
■Capiltaru 
Wafer 


Fig.  13. — The  various  fates  of  rain-water  which  falls  upon  the  soil.  It  may 
run  off  without  entering  the  soil;  it  may  be  evaporated  from  the  surface;  it  may 
enter  the  roots  and  be  transpired  from  the  leaves;  it  may  be  held  in  the  soil  by 
capillarity,  or  it  may  percolate  downward  to  the  water-table. 


or  hydrostatic  water.  This  level  is  known  as  the  wafer-table. 
Its  position  at  any  given  point  determines  the  height  at  which 
water  will  stand  in  a  well  dug  at  that  point,  and  its  distance 
below  the  surface  varies  from  place  to  place  and  is  subject  to 
much  fluctuation.  A  similar  saturated  condition  occurs  in  the 
upper  soil  layers  after  heavy  rains,  but  persists  there  for  only  a 
short  time.  When  water  has  percolated  downward  to  this  level 
it  is  often  beyond  the  reach  of  roots,  and  is  thus  quite  unavailable 
to  plants. 

Capillary  water  is  water  held  in  the  soil  by  the  force  of  capil- 
larity. Common  observation  teaches  that  when  an  object 
(such  as  one's  hand)  is  immersed  in  water  and  then  lifted  out 
again,  some  water  still  adheres  to  its  surface  in  a  thin  film,  or 
"wets"  the  object.  This  is  due  to  the  fact  that  there  is  greater 
attraction  between  the  surface  of  the  object  and  the  water  than 
is  exerted  by  the  force  of  gravity  or  the  cohesion  of  the  water 
particles  themselves.     Any  material   with   a  large  amount  of 


26 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


surface,  internal  or  external,  which  may  be  wetted  (such  as  a 
sponge,  blotting  paper  or  coarse  fabric)  will  therefore  hold  within 
itself,  when  thoroughly  soaked,  a  great  amount  of  water  which 
will  not  drain  out  under  gravity.  For  exactly  the  same  reason, 
much  of  the  water  entering  a  soil  will  fail  to  percolate  through  it 
but  will  instead  adhere  in  thin  films  to  the  surfaces,  very  great  in 
total  area,  which  are  presented  by  the  multitude  of  soil  particles. 
If  the  amount  of  rainfall  is  small,  all  of  it  may  thus  be  retained 


.  Air-SpacG 
.Water- Film 

•Rock-Parficle 


Fig.  14. — Section  through  soil,  much  enlarged.  A  capillary  water-film 
surrounds  each  particle  and  fills  the  narrow  spaces  between  particles.  The 
larger  spaces  are  occupied  by  air.     Much  enlarged. 

and  none  lost  through  percolation.  Each  particle  in  such  a  moist 
soil  is  covered  by  a  thin  layer  of  water  (Fig.  14).  The  films  about 
adjacent  particles  coalesce,  filling  the  minuter  spaces  and  lining 
those  that  are  larger,  and  a  continuous  film-system  is  thus  set 
up.  It  is  this  film  or  capillary  water  which  furnishes  plants 
with  the  great  bulk  of  their  water-supply.  One  of  the  important 
objects  in  manipulating  a  soil  is  to  increase,  by  one  means  or 
other,  this  water-holding  capacity,  and  thus  to  prevent  waste 
through  run-off  or  percolation. 

The  principle  of  capillarity  is  of  further  importance  in  deter- 
mining all  movements  of  water  in  the  soil  other  than  the  down- 
ward one  due  to  gravity.  The  familiar  fact  that  when  a  narrow 
glass  tube  is  placed  in  water,  the  water  will  rise  inside  the  tube 
to  a  point  somewhat  higher  than  its  level  outside,  is  due  to  the 
attraction  between  the  surface  of  the  glass  and  the  water,  an 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS  27 

attraction  which  is  sufficient  to  Hft  water  against  gravity. 
The  Hfting  force  will  be  proportional  to  the  exposed  surface  of 
the  tube,  and  therefore  where  the  volume  of  water  is  small  in 
relation  to  this  surface  (as  is  the  case  inside  the  tube)  the  water 
will  rise  somewhat  before  the  weight  of  the  lifted  column  counter- 
balances the  pull  exerted  by  the  surface  attraction.  Obviously, 
the  narrower  the  tube,  the  higher  the  column  of  water  will  rise, 
since  the  volume  of  liquid  to  be  lifted  will  be  smaller  in  proportion 
to  the  area  of  the  attracting  surface.     Thus,  in  any  material  the 


Fig  .15. — The  relation  between  the  size  of  soil  particles  and  the  amount  of 
surface  which  they  present.  Diagrams  of  sections  through  equal  volumes  of 
small  spheres  and  of  large  spheres.  The  total  surface  is  evidently  greater  where 
the  size  of  the  particles  is  less. 

structure  of  which  presents  a  great  amount  of  surface  surrounding 
small  but  communicating  tubes,  pores  or  other  narrow  spaces,  as 
in  blotting  paper,  lamp-wicks,  and  the  like,  water  will  evidently 
be  carried  to  a  considerable  distance  in  all  directions  by  capil- 
larity. Just  such  a  material  as  this  is  the  soil.  The  multitude 
of  its  tiny  particles,  packed  closely  together,  form  a  capillary 
system  which  is  able  to  carry  water  far.  This  water  tends  to 
surround  each  particle  in  a  thin,  capillary  film,  but  if  the  soil 
particles  are  very  coarse  the  film  cannot  pass  around  them,  and 
under  such  conditions  the  ascent  of  water  necessarily  stops. 
Water  moves  readily  within  the  films  and  when  those  at  the  top 
of  the  ascending  column  are  thinned  through  evaporation  or 
through  the  attraction  exerted  by  still  higher  and  un wetted 
surfaces,  the  films  below  are  drawn  upon,  and  water  passes  upward 
through  the  whole  system.  This  movement  continues  until  the 
weight  of  the  water  lifted  balances  the  surfaced  attraction  at  the 
top  of  the  column.     The  height  to  which  water  will  rise  by  capil- 


28 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


larity  is  dependent  chiefly  on  the  size  of  the  soil  particles;  for  the 
smaller  the  particles,  the  larger  will  be  their  surface  in  proportion 
to  the  spaces  between  them  (Fig.  15),  and  thus  the  higher  will  be 
the  rise  of  water  (Fig.  16).  In  ordinary  soil  this  rise  varies 
roughly  from  two  to  twenty  feet.  It  is  evident,  therefore,  that 
water  which  has  percolated  very  far  below  the  surface  ordinarily 
cannot  be  made  available  to  plants  again  through  capillary  ascent. 


Fig.  16. — Rise  of  water  by  capillarity.  In  the  two  glass  tubes  at  the  left,  the 
rise  of  water  is  much  higher  in  the  narrower  than  in  the  wider  one.  In  the  two 
chambers  at  the  right,  which  are  filled  with  spheres,  the  rise  of  water  is  much 
higher  where  the  spheres  are  smaller. 


In  most  soils  there  is  a  capillary  movement  of  water  toward 
the  soil  surface,  where  it  evaporates.  If  the  particles  at  the 
surface  are  very  close  together,  as  they  are  where  the  soil  has 
been  packed  down  or  where  a  crust  has  been  formed,  a  very 
efficient  capillary  system  is  produced  there  which  connects  the 
soil  surface  with  the  deeper  water-holding  layers,  and  thus  greatly 
hastens  the  loss  of  water  by  drawing  it  up  to  a  point  where  it  may 
be  evaporated  (Fig.  17).  An  important  purpose  of  tillage  is  to 
prevent  such  waste  of  water  by  breaking  up  the  capillary  system 
at  the  surface  and  forming  there  a  layer  of  loose,  coarse  material 
called  a  7nulch. 

Capillary  movement  of  water  is  by  no  means  always  vertical 
but  may  take  place  in  all  directions  within  a  soil,  just  as  ink 
spreads  in  all  directions  in  a  piece  of  blotting  paper.     This  move- 


THE  SOIL  AND  IT,S  IMPORTANCE  TO  PLANTS  29 

inent  tends  to  contimic  unl-il  tlu^  water  films  are  of  (hjuuI thickness 
throughout  the  entire  soil  mass,  causing  it  to  be  uniformly  moist. 
When  water  is  removed  at  any  particular  point,  as  by  surface 
evaporation  or  root  absorption,  it  is  therefore  drawn  thither 
from  all  other  points  until  equilibrium  is  restored. 


Fig.  17. — A  foot-pnnt  in  loose  soil.  A  vertical  slice  through  the  soil  under 
and  near  a  foot-pnnt,  showing  how  the  particles  which  were  under  the  foot  have 
been  pressed  together.  This  establishes  a  better  capillary  connection  with  the 
lower  soil  layers  and  causes  a  more  rapid  movement  of  water  to  the  surface,  thus 
often  making  the  foot-print  moist  while  the  soil  surface  around  it  is  dry. 

In  soils  which  have  lost  all  their  capillary  water  by  evaporation, 
there  still  remains  around  each  particle  an  exceedingly  thin 
film  of  hjgroscopic  water,  which  clings  so  tenaciously  that  it  may 
be  driven  off  only  by  subjecting  the  soil  to  a  high  temperature. 
When  the  air  is  very  dry,  this  water  is  present  in  minimum 
amount,  but  when  humidity  rises,  more  water  may  be  taken  up 
directly  from  the  air,  or  hygroscopically.  This  type  of  water  is 
removed  with  such  difficulty  from  the  soil  particles,  however, 
that  the  plant  is  able  to  obtain  little  or  none  of  it. 

Air. — Since  oxygen  is  essential  for  the  healthy  growth  of  ordi- 
nary plant  roots,  the  presence  in  the  soil  of  a  plentiful  supply  of 
air  is  a  matter  of  vital  importance.  If  the  spaces  between  the  soil 
particles  become  filled  with  water,  most  of  the  air  is  necessarily 
driven  out,  and  when  this  condition  of  saturation  is  long  main- 
tained, ordinary  plants  suffer.  We  have  seen,  however,  that  such 
excess  of  water  normally  passes  downward  by  percolation,  and  as 
it  does  so  the  soil  spaces  fill  again  with  air.  In  most  cultivated 
soils,  from  20  to  35  per  cent  of  the  volume  consists  of  air  spaces. 
The  composition  of  the  air  which  fills  these  is  often  somewhat 
different  from  that  of  the  atmosphere,  the  proportion  of  carbon 


30  BOTANY:  PRINCIPLES  AND  PROBLEMS 

dioxide  being  relatively  high.  Plowing  tends  to  increase  greatly 
the  air  content  of  a  soil,  since  the  structure  of  the  whole  mass  is 
loosened  and  the  crumbs  are  more  widely  separated.  A  soil  in 
this  condition  is  said  to  be  in  good  tilth.  Where  water  occurs 
only  in  capillary  form  much  air  is  present  in  the  larger  spaces, 
and  such  a  state  of  the  soil  is  therefore  clearly  the  most  favor- 
able for  plant  growth  since  then,  and  then  only,  is  a  plentiful 
supply  of  water  combined  with  a  plentiful  supply  of  oxygen. 

Organic  Matter. — All  rich  soils  contain  a  considerable  amount 
of  material  derived  from  the  dead  bodies  of  organisms,  particu- 
larly of  plants.  Roots  which  die  and  remain  in  the  soil,  and 
leaves  and  other  plant  parts  which  fall  on  the  soil  surface,  ^re  the 
sources  from  which  this  organic  matter  is  mainly  derived  in  nature. 
In  the  practice  of  agriculture  it  is  increased  in  amount  by  various 
artificial  means.  After  entering  the  soil  it  soon  begins  to  undergo 
decomposition,  and  for  the  most  part  is  finally  broken  down  into 
simple  end-products — carbon  dioxide,  water,  and  ammonia.  As 
this  organic  material  decays  it  becomes  characteristically  dark  in 
color  and  undergoes  a  series  of  complex  chemical  changes.  In 
this  condition  it  is  known  by  the  general  name  of  humus. 

Humus  is  of  importance  to  plants  in  many  ways.  It  improves 
the  physical  condition  of  the  soil,  for  because  of  its  coarse  and 
fragmentary  character  it  tends  to  separate  the  particles  and  thus 
to  increase  materially  the  air-content  of  the  soil.  Since  humus 
absorbs  water  readily,  its  presence  also  adds  to  a  soil's  water- 
holding  capacity.  The  decomposition  of  humus  liberates  certain 
nutrient  materials,  notably  an  abundant  supply  of  nitrogen  com- 
pounds, which  ultimately  become  available  to  plants.  Humus  is 
also  the  seat  and  food-supply  of  the  soil  bacteria,  minute  organ- 
isms which  are  indispensable  in  plant  nutrition.  Any  treatment 
of  the  soil  which  will  increase  its  humus  content  will  therefore 
tend  to  increase  its  productivity,  and  whatever  decreases  the 
humus  content  will  impoverish  the  soil. 

Dissolved  Substances. — Soil  water  is  by  no  means  pure  water 
but  carries  dissolved  within  it  a  great  variety  of  substances. 
Anything  which  is  to  be  taken  in  by  the  roots  of  plants  must  be  in 
solution,  and  it  is  consequently  obvious  that  these  dissolved 
substances  are  the  only  portion  of  the  soil,  aside  from  water  itself, 
which  is  directly  available  as  nutrient  material  for  plants. 
Their  origin  and  chemical  composition  are  therefore  of  much 
importance  botanically. 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS  31 

The  solvent  power  of  soil  water  is  increased  by  the  presence 
within  it  of  carbon  dioxide,  liberated  in  the  respiration  of  plant 
roots  and  of  the  lower  organisms.  Thus  reinforced,  water  not 
only  attacks  the  surfaces  of  the  rock  particles  but  absorbs  any 
soluble  material  which  may  appear  in  the  humus  or  as  a  product 
of  bacterial  activity. 

There  is  a  great  variety  of  substances  present  in  the  soil 
solution,  and  we  know  from  chemical  analyses  of  the  ash*  of 
plants  that  very  many  of  them  may  be  taken  into  the  plant  body. 
Compounds  of  nitrogen,  sodium,  potassium,  calcium,  magnesium, 
iron,  manganese,  aluminum,  phosphorus,  sulphur,  chlorine,  and 
silicon  are  commonly  absorbed  by  the  roots,  and  many  others 
may  be  taken  up  occasionally.  Certain  of  these  elements  are 
far  more  important  to  the  plant  than  others,  however,  and  it  has 
been  clearly  proven  by  experiment  that  seven  are  essential  for 
normal  plant  growth:  Sulphur,  phosphorus,  calcium,  magne- 
sium, potassium,  iron,  and  nitrogen.  The  actual  amount  of  these 
mineral  nutrients  taken  up  by  the  plant  is  exceedingly  small  in 
proportion  to  the  size  of  the  plant  body,  but  in  the  activities 
of  protoplasm  each  plays  a  very  necessary  part,  and  a  soil  which  is 
deficient  in  any  one  of  them  will  be  unable  to  support  vegetation 
successfully. 

The  removal  of  large  amounts  of  nutrient  materials  from  agri- 
cultural soils,  in  the  form  of  crops  and  in  other  ways,  reduces  the 
available  supply  of  certain  chemical  elements,  notably  nitrogen, 
phosphorus,  and  potassium,  to  such  an  extent  that  a  fresh  supply 
must  be  returned  to  the  soil  if  abundant  plant  growth  is  to  be 
maintained  permanently  thereon.  This  necessitates  the  common 
practice  of  adding  to  the  soil  various  tj^pes  of  fertilizers  which 
renew  the  supply  of  essential  salts  there  available  to  plants. 

Organisms. — Aside  from  its  service  as  a  medium  for  the  root- 
growth  of  higher  plants,  the  soil  provides  a  dwelling-place  for  a 
great  variety  of  other  organisms,  whose  activities  have  a  profound 
effect  on  the  composition  of  the  soil  and  on  the  processes  which  go 
on  therein.  Rodents,  insects,  and  angleworms  all  modify  the 
physical  character  of  the  soil  by  their  abode  within  it.  Those 
most  minute  and  lowly  of  living  things,  however,  which  we 
group  together  as  micro-organisms  are  of  far  greater  importance, 

*  The  ash  of  plants  is  the  residue  left  after  complete  combustion  of  the 
plant  tissues,  and  an  analysis  of  it  indicates  the  amount  and  character  of 
mineral  substances  present  in  the  plant. 


32  BOTANY:  PRINCIPLES  AND  PROBLEMS 

for  experiment  has  shown  that  without  their  presence  the  soil 
would  soon  become  unfit  to  support  a  vegetation  of  higher  plants. 
Most  notable  among  these  micro-organisms  are  the  Bacteria 
(Fig.  18),  tiny,  single-celled  plants  which  lack  the  green  pigment 
chlorophyll.  Many  of  these — the  bacteria  of  decay — decompose 
the  complex  organic  substances  found  in  humus  into  such  simple 
end-products  as  carbon  dioxide,  water,  and  ammonia,  thus 
releasing  great  quantities  of  nutrient  materials  which  would 
otherwise  be  locked  up  and  useless  in  dead  bodies  of  animals 


^6  e® 


B 


^' 


Fig.  18. — Some  important  soil  bacteria.  A,  nitrite  bacteria,  Nitromonas; 
X  2000.  B,  nitrate  bacteria,  Nitrobacter;  X  2000.  C,  a  common  decay-produc- 
ing organism,  Bacterium  mycoides;  X  1500.  D,  nitrogen-fixing  bacteria,  Rhizo- 
bium  leguminosarum;   X  750. 

and  plants.  Still  other  bacteria  in  the  soil  cause  chemical  changes 
of  various  sorts  there,  the  results  of  which  are  of  great  moment 
to  the  higher  plants.  Notable  among  these  are  bacteria  con- 
cerned with  the  transformations  of  nitrogen  and  its  compounds, 
for*  through  their  activity  alone  is  the  available  supply  of  this 
necessary  element  maintained  in  the  soil.  The  continual  circula- 
tion of  nitrogen  through  its  various  successive  stations  in  organ- 
isms, air,  and  soil  is  known  as  the  Nitrogen  Cycle  (Fig.  19). 
Complex  nitrogenous  substances  returned  to  the  soil  in  the  bodies 
of  dead  animals  and  plants  are  broken  down  by  the  bacteria  of 
decay  into  simpler  compounds,  which  are  finally  reduced  to 
ammonia.  Since  most  plants  can  use  nitrogen  only  when  it 
occurs  in  the  form  of  nitrate  salts,  however,  this  ammonia  is  not 
directly  available  to  them  but  must  first  be  converted  into  nitrate 
salts  through  the  process  of  nitrification.     This  is  carried  on  by 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS 


33 


two  types  of  nitrifying  bacteria;  the  nitrite  bacteria,  which  change 
ammonia  to  nitrites,  and  the  nitrate  bacteria,  which  in  turn 
convert  nitrites  into  nitrates.  In  this  form  nitrogen  is  readily 
absorbed  and  assimilated  by  plants,  and  is  ultimately  returned 
to  the  soil  again  in  the  bodies  of  plants  or  animals,  thus  complet- 
ing the  cycle.     Through  the  activity  of  another  group  of  these 


Dcnitrificafion 
The    Soil  Ammonia 


iNJtrate 

Salts 


Ni+nfmnc 
Bacterio 


Fig.   19.— The  Nitrogen  Cycle. 


minute  organisms,  certain  of  the  seed  plants  are  also  able  to  take 
advantage  of  the  enormous  supply  of  nitrogen  in  the  atmosphere, 
which  is  ordinarily  quite  unavailable.  These  are  the  nitrogen- 
fixing  bacteria.  They  are  present  in  most  soils  and  cause  the 
development  of  the  tubercles  or  nodules  usually  found  on  the 
roots  of  plants  belonging  to  the  Legume  family  (Fig,  20),  which 
includes  beans,  peas,  clover,  alfalfa,  and  similar  plants.  These 
bacteria  are  able  to  absorb  the  free  gaseous  nitrogen  of  the  air 
and  to  build  it  into  nitrogenous  compounds  in  their  bodies, 
whence  it  ultimately  becomes  available  to  the  particular  plant 
on  the  roots  of  which  the  bacteria  grew.  Without  drawing  at  all 
upon  the  nitrogen  compounds  in  the  soil,  a  leguminous  plant 
is  consequently  able  to  acquire  an  abundant  supply  of  this  impor- 
tant element. 


34 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


In  the  case  of  many  species  of  plants,  particularly  those  which 
grow  in  forests  or  other  situations  rich  in  humus,  thread-like 
filaments  of  fungi  are  intimately  associated  with  the  smaller 
roots,  entering  their  outer  tissues  and  surrounding  the  root  with 
a  web-like  jacket  of  fungus  threads.  These  very  largely  take  the 
place  of  root-hairs  and  aid  the  plant  in  absorbing  water  and 


Fig.  20. — Root  tubercles.  A  part  of  the  root  system  of  a  leguminous  pkint, 
showing  the  characteristic  tubercles  upon  the  roots,  caused  by  the  presence  of 
nitrogen-fixing  bacteria. 


nutrient  material  from  the  soil;  and  the  fungus,  as  well,  is 
evidently  benefited  by  such  relationship.  This  root-fungus 
association  is  known  as  a  mycorrhiza.  Certain  plants  have  be- 
come so  dependent  in  this  way  upon  particular  species  of  fungi 
that  they  cannot  thrive  when  these  fungi  are  absent. 

In  conclusion,  we  may  emphasize  again  the  extreme  complexity 
of  the  soil  and  the  vital  significance  to  plants  of  its  composition 
and  of  the  changes  which  go  on  within  it.  The  study  of  this 
remarkable  material  has  required  the  collaboration  of  almost 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS  35 

all  of  the  sciences,  but  we  still  lack  a  precise  knowledge  of  many 
of  its  aspects  and  fail  to  understand  clearly  the  manner  in  which 
it  affects  the  life  of  plants  growing  in  it. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

53.  Through  what  processes  does  bare  rock  gradually  become  covered 
with  soil? 

54.  Is  soil  always  formed  by  the  rock  lying  directly  under  it?  What 
was  the  origin  of  the  soil  in  this  region? 

55.  In  what  various  ways  is  soil  being  impoverished  or  "used  up?" 

56.  In  what  various  ways  is  soil  being  replenished? 

57.  What  is  the  surface  of  a  bit  of  gravel  which  is  in  the  form  of  a 
solid  cube,  the  sides  of  which  are  1  cm.  long?  What  would  be  the 
surface  of  this  bit  of  gravel  if  the  cube  were  cut  up  into  smaUer  cubes  with 
sides  of  1  mm.  each?  What  conclusion  can  you  draw  from  this  as  to 
the  importance  of  the  size  of  soil-particles? 

58.  What  disadvantages  to  plants  are  there  in  a  soil  where  the 
particles  are  very  small?  What  disadvantages  where  they  are  large 
and  coarse? 

59.  Why  are  soils  in  which  the  particles  are  small  usually  more  pro- 
ductive than  soils  in  which  the  particles  are  large? 

60.  It  is  important  to  have  the  soil  very  loose  and  "mehow"  if  such 
"root-crops"  as  potatoes,  carrots,  beets,  and  turnips  are  to  yield  heavily. 
Explain. 

61.  The  surface  of  land  after  plowing  is  a  few  inches  higher  than  it  is 
before  plowing.     Why? 

62.  In  what  respect  is  the  cutting  off  of  forests  bad  for  the  soil? 

63.  Is  a  gentle  shower  or  a  hard,  beating  rain  better  for  the  soil  and 
for  plants  growing  thereon?     Explain. 

64.  It  is  harmful  to  the  soil  to  till  (or  "work")  it  immediately  after  a 
rain  or  while  the  soil  is  very  wet.     Why? 

65.  Why  is  extensive  removal  of  forests  in  a  region  often  followed  by 
floods  there? 

66.  What  various  means  of  soil  treatment  can  you  suggest  for  insuring 
that  the  largest  possible  proportion  of  the  rainfall  is  utilized  by  plants 
growing  on  this  soil? 


36  BOTANY:  PRINCIPLES  AND  PROBLEMS 

67.  Which  will  absorb  and  hold  a  larger  amount  of  water:  Clay  or 
sand?     Why? 

68.  Will  the  rise  of  water  by  capillarity  be  higher  in  sand  or  in  clay? 
Why? 

69.  Which  is  better  for  plant  growth  in  a  dry  season,  a  clay  soil  or 
a  sandy  one?     Why? 

70.  A  layer  of  coarse  gravel  a  few  feet  below  the  surface  of  the  soil 
makes  the  soil  above  it  much  drier  than  similar  soil  not  underlain  by 
gravel.     Why? 

71.  Why  does  the  surface  of  soil  which  has  been  perfectly  dry  during 
the  day  often  turn  moist  at  night? 

72.  Why  is  the  surface  of  the  soil  in  arid  regions  often  caked  with  a 
crust  of  salts? 

73.  Why  is  a  well-forested  region  apt  to  suffer  less  from  drought  than 
one  with  no  forests? 

74.  Which  will  generally  be  colder,  a  soil  with  fine  particles  or  one 
with  coarse?     Why? 

75.  A  potted  plant  will  dry  out  the  soil  in  its  pot  very  uniformly. 
Explain. 

76.  Is  it  better  to  water  house  plants  by  pouring  water  over  the  soil 
or  by  letting  the  pots  stand  in  water  for  a  little  while?     Why? 

77.  Explain  exactly  ivhy  a  mulch  on  the  soil  surface  reduces  loss  of 
water  from  the  soil  through  evaporation. 

78.  What  basis  in  fact  is  there  for  the  saying,  "Water  your  garden 
with  a  rake"? 

79.  Why  is  it  advisable  to  scratch  over  a  garden  with  hoe  or  rake 
as  soon  after  a  rain  as  the  soil  is  workable? 

80.  Why  is  it  advisable  to  plow  as  early  in  the  spring  as  the  soil  is 
workable? 

81.  Why  is  it  important  to  water  a  plant  before  it  is  transplanted? 

82.  Give  three  reasons  for  pressing  the  soil  firmly  about  the  roots 
of  a  plant  after  it  has  been  transplanted. 

83.  After  a  plant  has  been  transplanted,  it  is  well  to  scratch  over  the 
ground  around  it  with  a  hoe  or  rake.     Why? 

84.  What  advantage  is  gained  by  pressing  down  the  soil  above  seods 
after  planting  them? 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS  37 

85.  After  sowing  seed  for  certain  crops,  a  farmer  often  rolls  the  soil 
with  a  heavy  roller.     What  is  gained  by  this? 

86.  "Why  is  irrigation  necessary  in  some  regions  l)ut  not  in  others? 

87.  What  is  the  l)est  time  of  day  to  water  a  garden?     Why? 

88.  Why  is  it  necessary  to  drain  wet  land  before  ordinary  crops  can 
be  grown  thereon? 

89.  In  what  ways  is  the  air  in  the  soil  constantly  being  changed  and 
renewed? 

90.  Of  what  use  is  the  air  in  the  soil  in  addition  to  providing  oxygen  for 
plant  roots? 

91.  Give  two  reasons  why  the  formation  of  a  "crust"  on  the  soil 
surface  is  harmful  to  plants. 

92.  Is  it  better  to  water  house  phmts  frequently  antl  lightly  or  infre- 
quently and  heavily?     Why? 

93.  How  is  the  supi)ly  of  humus  maintained  in  soils  which  are  not 
under  cultivation? 

94.  Dark-colored  soil  is  usually  richer  than  light-colored  soil. 
Explain. 

95.  Why  is  soil  from  low  land  usually  darker  in  color  than  soil  from 
a  side  hill? 

96.  The  continual  addition  to  the  soil  of  no  other  fertilizer  than 
the  ordinary  "commercial"  fertilizers  is  generally  found  to  be  harm- 
ful.    Why? 

97.  What  conditions  are  there  under  which  the  addition  of  humus  to 
the  soil  might  be  actually  injurious  to  plants? 

98.  How  would  you  prove  that  a  particular  element  is,  or  is  not,  essen- 
tial to  plant  life? 

99.  Most  commercial  fertilizers  contain  nitrates,  phosphoric  acid, 
and  potash.     Why? 

100.  A  good  crop  of  corn,  wheat  or  potatoes  removes  about  50  pounds 
of  nitrogen  per  acre  from  the  soil.  Nitrogen  composes  about  four-fifths 
of  the  weight  of  the  atmosphere.  At  this  rate  of  removal  by  crops, 
how  many  years  would  the  nitrogen  in  the  air  over  an  acre  last,  provided 
that  it  could  be  made  available  to  the  plants  growing  there? 

101.  Name  several  ways  in  which  chemical  substances  essential  to 
plant  growth  are  wasted  in  modern  civilization.  What  methods  can 
you  suggest  to  prevent  some  of  this  waste? 


38  BOTANY:  PRINCIPLES  AND  PROBLEMS 

102.  Why  are  wood  ashes  so  valuable  as  fertilizer? 

103.  Why  do  sewers  often  become  clogged  with  roots? 

104.  What  two  important  contributions  to  human  welfare  are  made 
by  the  bacteria  of  decay? 

105.  What  various  organisms  living  in  the  soil  may  be  harmful  to 
plants? 

106.  What  is  the  chief  importance  of  angleworms  in  the  growth  of 
plants? 

107.  Plants  native  to  the  woods  will  often  fail  to  thrive  when  trans- 
planted to  a  garden,  even  though  the  soil  there  is  rich  and  the  conditions 
of  shade  and  temperature  are  much  like  those  in  the  forest.  Can  you 
suggest  a  reason  for  this  difficulty? 

108.  At  what  point  in  the  nitrogen  cycle,  and  in  what  form,  is  loss  of 
available  nitrogen  most  apt  to  occur? 

109.  Manure  left  freely  exposed  to  the  air  will  lose  much  of  its  fertiliz- 
ing value.     Why? 

110.  For  many  plants,  rather  old  manure  is  better  than  that  which 
is  absolutely  fresh.     What  reason  can  you  suggest  for  this? 

111.  Plants  which  have  very  deep  roots  (such  as  certain  weeds  and 
cover-crop  plants)  are  sometimes  of  advantage  to  crops  which  are 
subsequently  grown  on  the  soil.     Why? 

112.  A  "cover-crop"  is  a  crop  (such  as  rye)  planted  in  late  summer  or 
early  fall  which  grows  up  enough  to  cover  the  ground  before  winter. 
What  are  the  advantages  of  planting  such  a  crop? 

113.  Why  is  flood-plain  or  "river-bottom"  soil  usually  very  produc- 
tive? 

114.  In  what  ways  may  the  productivity  of  a  soil  be  diminished  other 
than  by  the  removal  of  crops  grown  upon  it? 

115.  It  has  long  been  recognized  that  land  which  is  left  uncropped 
or  "fallow"  for  a  few  years  will  prove  to  be  more  productive  after- 
ward.    How  do  you  explain  this? 

116.  In  what  ways  is  the  productivity  of  a  soil  maintained  in  nature, 
before  it  becomes  the  seat  of  agriculture? 

117.  What  other  functions  may  the  addition  of  fertilizer  to  the  soil 
perform  aside  from  that  of  providing  nutrient  materials  for  plants? 


THE  SOIL  AND  ITS  IMPORTANCE  TO  PLANTS  30 

118.  State  at  least  three  advantages  which  are  gained  by  plowing 
the  soil. 

119.  Cultivating  the  soil  around  growing  crop  plants  (by  hoeing  or 
otherwise)  is  not  necessary  or  advantageous  after  the  crop  has  covered 
the  ground  fairly  well.     Explain. 

120.  How  can  soil  in  a  state  of  nature,  entirely  without  cultivation, 
support  plant  life  so  well? 

121.  Give  several  reasons  for  the  desirability  of  keeping  the  soil 
around  a  young  fruit  tree  well  "mulched." 

122.  Vegetable  growers  often  sterilize  by  steam,  formalin  or  other 
means,  the  soil  in  the  greenhouse  beds  where  they  are  to  grow  their 
young  plants  for  later  transplanting  outside.  What  advantages  has 
this  process?  What  disadvantages  would  it  have  if  applied  in  the 
field? 

123.  Farmers  sometimes  build  a  big  bonfire  over  the  spot  where  they 
are  going  to  start  their  young  plants  of  cabbage,  tomato,  and  similar 
crops.     What  two  advantages  are  gained  by  this? 


REFERENCE  PROBLEMS 

12.  What  is  meant  by  tillage? 

13.  What  is  irrigation  and  how  is  it  practiced? 

14.  Soils  which   are  poorly  aerated  are  apt  to  be   "sour."     Explain. 

15.  Distinguish  between  animal  manures,  green  manures,  and  com- 
mercial fertilizers,  and  state  the  advantages  in  the  use  of  each. 

16.  What  is  the  chemical  formula  of  atmospheric  nitrogen?  of  ammonia? 
of  potassium  nitrate?  of  some  complex  nitrogenous  compound? 

17.  May  organic  substances  dissolved  in  the  soil  solution  enter  directly 
into  ordinary  green  plants  and  be  used  by  them  as  food  ?     Explain. 

18.  Give  an  example  of  a  crop  plant  which  thrives  best  on  alkaline  soil; 
one  that  thrives  best  on  acid  soil.  Which  of  these  two  soil  types  do  most 
crop  plants  favor? 

19.  About  how  many  bacteria  are  usually  present  in  1  c.c.  of  rich  soil? 

20.  What  is  "rotation  of  crops  "  and  what  is  its  value? 

21.  Give  the  derivation  of  each  of  the  following  terms  and  explain  in  what 
way  it  is  an  appropriate  one: 

Capillarity  Humus  Mycorrhiza 


CHAPTER   IV 

THE  ROOT  AND  ITS  FUNCTIONS 

The  portion  of  the  plant  most  intimately  related  to  the  soil 
is  the  root.  This  organ  has  two  major  functions — to  anchor  the 
plant  firmly  and  to  absorb  water  and  certain  important  nutrient 
materials  from  the  soil.     Beyond  this,  the  root  often  serves  as  a 


Fig.  21. — A  fibrous  root-system  (Grass).     The  roots  are  all  rather  slender  and 
much  branched. 

storage    reservoir   for   food,    and   may   perform   various    other 
functions. 

External  Structure. — The  most  common  type  of  root  is  a  rather 
slender  and  profusely  branched  structure,  penetrating  the  soil  in 

40 


THE  ROOT  AND  ITS  FUNCTIONS 


41 


all  directions  and  forming  a  fibrous  root-systoni  (Fig.  21).  Its 
advantages  for  anchorage  and  absorption  ai'e  obvious.  Somewhat 
less  common  arc  types  which  poss(\ss  a  single  main  root  or  iap- 


FiG.  22. — A  tap-root  system  (Dandelion).     From  the  tap-root  a  large  number  of 
smaller,  lateral  roots  arise. 


root,  penetrating  deeply  into  the  soil  and  much  stouter  than  the 
lateral  roots  which  arise  from  it  (Fig.  22).  Tap-roots  lend  them- 
selves readily  to  storage  purposes  and  frecjuenfly  ])ecome  large 
and  fleshy.     The  root-systems  of  many  jilants  are  internuMlinte 


42 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


between  these  two  main  types.  Others  may  sometimes  depart 
radically  from  the  normal  forms  in  response  to  certain  special 
and  unusual  functions  which  they  have  assumed. 

The  Absorbing  Region.^ — Absorption  of  water  and  nutrient 
material  is  carried  on  only  by  the  j^ounger  portions  of  the  root, 
near  its  tip.     The  very  tip  itself  is  covered  with  a  sheathing  root- 


FiG.  23. — Tip  of  a  root,  showing  root-cap  (A),  growth  zone  (B),  and  root-hair 
zone  (C).  (From  Ganong,  "Textbook  of  Botany",  copyrighted  by  the  Macmillan 
Company.     Reprinted  by  permission) . 

cap  of  cells,  which  protects  the  delicate  underlying  tissues  as  the 
root  pushes  its  way  through  the  soil  (Fig.  71).  Back  of  this  is  a 
short  region  of  growth,  the  only  place  where  elongation  of  the 
root  occurs.  Behind  this,  in  turn,  is  the  absorbing  region,  a 
somewhat  longer  zone  the  surface  of  which  is  covered  with 
thousands  of  exceedingly  delicate  filaments,  the  root-hairs  (Fig. 
23).  Each  hair  is  an  elongated  projection  growing  out  from  one 
of  the  surface  cells  of  the  root  (Fig.  24),  its  sap-cavity  and  lining 
of  cytoplasm  being  continuous  with  those  of  the  root-cell  of 
which  it  forms  a  part  (Fig.  25) .     The  root-hair  may  reach  a  length 


THE  ROOT  AND  ITS  FUNCTIONS 


43 


of  several  millimeters  and  forces  its  way  into  the  minute  crevices 
of  the  soil,  thus  coming  into  most  intimate  contact  with  the  soil 
particles  (Fig.  26).  Through  the  enormous  surface  which  these 
root-hairs  expose  to  the  soil,  absorption  of  water  and  mineral 


Fig.  24. — Root-hairs  and  their  relation  to  the  root.  A  transverse  section 
across  the  root-hair  zone,  showing  the  attachment  of  the  hairs.  In  the  root  may 
be  distinguished  the  fibro-vascular  cylinder  (in  the  center),  surrounded  by  the 
cortex. 


Cy+oplc 


Nucleus 


Fig.  25. — A  root-hair.     Section  through  a  typical  root-hair,  showing  its  various 
structures  and  its  relation  to  the  surface  cells  of  the  root. 


salts  takes  place.  Root-hairs  are  generally  short-lived,  dying 
away  as  the  corky  bark  begins  to  appear.  A  root-hair  zone  of 
fairly  constant  length  thus  follows  behind  the  growing  root-tip, 
new  hairs  appearing  in  its  younger  portion  to  replace  the  oldest 
ones,  which  arc  continually  dying  away. 


44 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


The  Plant  Cell. — The  root-hair  (inchiding  its  basal  portion)  is  a 
plant  cell.  Since  a  knowledge  of  the  structure  and  functions  of 
cells  is  obviously  essential  if  we  are  to  understand  how  the  root- 
hair  is  constructed  and  does  its  work,  or,  indeed,  how  any  other 
part  of  the  plant  is  put  together  and  functions,  it  will  be  necessary 
at  this  point,  bofoio  wo  discuss  the  physiology  of  the  root,  to 


\ 


■r^^^ 


Aim. 


Fig.  28. — Diagrammatic  section  through  a  portion  of  a  young  root  and  the 
adjacent  soil,  showing  two  root-hairs  forcing  their  way  between  the  soil  particles. 
Rock  particles,  water  films,  and  air  spaces  in  the  soil  are  .shown.      Much  enlarged. 

describe  briefly  some  of  the  more  important  characteristics  of 
cells  in  general  and  of  plant  cells  in  particular. 

We  have  already  spoken  of  that  remarkable  living  material 
which  is  called  protoplasm,  the  seat  of  all  the  various  activities 
which  are  maintained  in  animals  and  plants,  and  the  only  portion 
of  their  bodies  which  is  truly  alive.  Physically,  protoplasm  is 
a  thin,  jelly-like,  colloidal  substance,  but  its  minute  structure  is 
not  clearly  known.  Chemically,  it  is  a  mixture  of  very  complex 
proteins  and  is  thus  composed  chiefly  of  carbon,  oxygen,  hydrogen, 
and  nitrogen.  Protoplasm  is  the  "physical  basis"  of  all  life  and 
the  most  extraordinary  material  known  to  man. 

The  protoplasm  of  the  plant  body  is  not  a  directly  continuous 
mass  but  is  broken  up  into  minute  parts,  the  cells  or  protoplasts, 
each  of  which  is  a  distinct  and  more  or  less  independent  unit, 
possessing  a  definite  structure  and  carrying  on  within  itself  a 
variety  of  physiological  processes  (Fig.  27).  Around  the  cell  the 
protoplasm  secretes  a  dead  cell-wall  composed  of  the  characteristic 


THE  ROOT  AND  ITS  FUNCTIONS 


45 


plant  substance  cellulose.  This  is  firm  in  texture  but  easily 
penetrated  by  water.  The  protoplasm  of  the  cell  has  two  distinct 
portions — the  nucleus,  a  dense,  somewhat  spherical  body  which 
appears  to  be  the  directive  center  for  the  cell's  activities;  and  the 
cytoplasm,  thinner  and  more  watery  in  texture,  which  lines  the 
inner  surface  of  the  wall  in  a  tenuous  laj^er  and  is  bounded, 


Fig.  27. — A  typical  plant  cell.  Diagrammatic  drawing  .showing  the  cen- 
tral sap-cavity,  surrounded  by  a  layer  of  cytoplasm  which  lines  the  cell-wall. 
Embedded  in  the  cytoplasm  is  the  nucleus.  Parts  of  the  cell-walls  of  six  adjacent 
cells  are  also  shown. 


without  and  within,  by  a  very  delicate  membrane.  Embedded 
in  the  cytoplasm  frequently  appear  small,  somewhat  denser 
bodies,  the  plastids.  These  perform  special  functions,  such  as 
carrying  on  the  manufacture  of  fr  )d,  building  up  starch-grains  or 
producing  colors.  In  many  case^  it  has  been  shown  that  the 
cytoplasm  is  not  passive  and  immobile,  but  that  within  it  a  slow, 
streaming  movement  often  takes  place.  In  mature  cells,  the  entire 
central  portion  of  the  cell  is  occupied  by  a  sap-cavity  or  vacuole, 
filled  with  water  in  which  various  substances  are  dissolved,  and 
surrounded  externally  by  the  layer  of  cytoplasm.  A  typical 
plant  cell  may  thus  perhaps  be  likened  roughly  to  an  inflated 
football  or  basketball,  the  firm  leathery  covering  corresponding 
to  the  cell-wall;  the  thin  inner  bladder  of  rubber,  tightly  pressed 
against  it,  to  the  layer  of  cytoplasm;  and  the  air-space  to  the 
sap-cavity.  A  comparison  to  an  automobile  tire,  with  its  stout 
shoe  or  casing,  its  delicate  inner  tube,  and  its  central  air-cavity, 
might  also  be  made. 

Cells  are  normally  very  small  objects,  averaging  about  .01 
mm.  in  diameter,  and  varying  widely  in  shape  and  character 
according  to  the  function  which  each  performs,  whether  this  be 
support,  absorption,  conduction,  storage,  protection,  food-manu- 


46  BOTANY:  PRINCIPLES  AND  PROBLEMS 

factuie,  growth,  or  reproduction.  The  plant  body  is  composed  of 
a  multitude  of  cells,  bound  firmly  together  by  cementing  sub- 
stances to  form  an  entire,  coherent  organism.  As  we  consider 
the  various  tissues  and  organs  in  detail,  we  shall  have  occasion 
to  describe  the  particular  characteristics  which  their  cells  display. 
Internal  Structure  of  Roots. — Internally,  roots  show  a  very 
marked  structural  differentiation.  In  a  young  root,  three  main 
cell-systems  or  tissues  are  distinguishable — the  epidermis,  the 
cortex,  and  the  fibro-vascular  cylinder  (Fig.  28). 

too 
6C 

Dc. 

Fig.  28. — Transverse  section  of  the  fibro-vascular  cylinder  of  a  young  root. 
The  bundles  of  thick-walled  wood  cells  alternate  with  bundles  of  thin-walled 
bast  cells.  The  whole  is  surrounded  by  an  endodermis,  outside  of  which  is  the 
cortex  of  the  root.     (From  De  Bary). 

The  epidermis  of  the  root,  like  that  of  all  other  plant  organs, 
is  a  single  layer  of  cells  in  thickness.  These  cells  are  normally 
protective  in  function,  but  in  the  root-hair  zone  many  of  them 
produce  on  their  outer  surface  a  characteristic  projection,  the 
root-hair  itself. 

The  cortex  lies  just  under  the  epidermis  and  is  of  varying 
thickness.  In  the  young  root,  its  cells  serve  to  transmit  water 
and  dissolved  substances  from  root-hair  to  fibro-vascular  cylinder, 
and,  in  the  older  roots,  to  store  food.  Most  of  the  fleshy  portion 
of  storage-roots  consists  of  enlarged  cortex.  The  innermost 
layer  of  cortical  cells  is  often  especially  modified  and  is  then 
known  as  the  endodermis. 


THE  HOOT  AND  ITS  FUNCTIONS 


47 


The  fibro-vaseular  cylinder  occupies  the  core  of  the  root, 
furnishing  mechanical  strength  and  serving  as  a  highway  for  con- 
duction. As  in  other  organs  of  the  plant,  it  is  composed  of  two 
main  tissues,  the  wood  or  xylem  and  the  hast  or  phloevi.  The 
wood,  which  forms  the  central  axis  of  this  cylinder,  is  usually 
star-shaped  in  cross  section  and  is  composed  for  the  most  part 
of  thick-walled  and  much  elongated  dead  cells,  the  walls  of  which 
have  become  woody.  It  provides  rigidity  for  the  root  and 
conducts   upward   the   water   and   dissolved   substances   which 


Fig.  29. — Diffusion  of  a  dissolved  substance.  Diagram  representing  the 
outward  diffusion  of  molecules  which  are  being  dissolved  from  the  surface  of  a 
soluble  substance  immersed  in  water. 

enter  from  the  soil.  Between  the  points  of  the  star  are  patches 
of  bast,  formed  of  thin-walled  cells  which  transport  manufactured 
food  upward  or  downward. 

In  older  roots  the  fibro-vascular  cylinder,  particularly  as 
to  its  woody  portion,  increases  greatly  in  thickness  through  the 
activity  of  a  growing  zone  or  cambium,  just  as  does  the  stem; 
and  a  corky  bark  is  usually  developed  on  the  outside. 

Diffusion  and  Osmosis. — The  most  important  function  of 
the  root  is  to  absorb  from  the  soil  the  water  and  mineral  sub- 
stances needed  for  the  plant's  life  and  growth.  This  involves 
the  physical  processes  of  diffusion  and  osmosis,  a  consideration 
of  which  is  necessary  before  we  can  understand  clearly  this 
primary  activity  of  the  root. 

Diffusion.- — Diffusion  may  be  defined  as  the  tendency  of  any 
substance,  when  it  occurs  as  a  gas  or  in  solution,  to  become 


48  BOTANY:  PRINCIPLES  AND  PROBLEMS 

evenly  distributed  throughout  the  whole  space  available  to  it 
by  moving  from  points  of  greater  to  points  of  lesser  concentration. 
Its  operation  is  familiar  in  the  diffusion  of  odors,  for  the  minute 
particles  given  off  by  any  strongly  scented  substance  will  move 
outward  rapidly,  even  in  perfectly  still  air,  and  will  soon  become 
equally  distinguishable  in  all  directions  from  their  point  of 
origin.  Two  gases  liberated  within  a  closed  space  soon  diffuse 
throughout  its  whole  extent  and  become  thoroughly  mixed. 
In  the  same  way,  a  crystal  of  salt  dissolved  in  a  vessel  of  water 
will  in  time  have  its  molecules  dispersed  so  uniformly,  even  though 
the  liquid  is  free  from  moving  currents,  that  samples  taken  from 
any  part  of  the  contents  of  the  vessel  will  be  salt  solutions  of 
exactly  the  same  strength  (Fig.  29).  This  constant  tendency 
toward  diffusion  is  explained  by  the  fact  that  in  gases  and  hquids 
the  molecules  are  in  very  active  movement,  continually  striking 
against  one  another  and  rebounding.  There  are  obviously 
fewest  collisions,  and  thus  most  frequent  opportunity  for  unob- 
structed movement,  in  those  directions  where  there  are  fewest 
molecules,  and  in  such  directions  a  dispersal  of  the  molecules 
therefore  necessarily  takes  place  until  they  are  present  every- 
where in  uniform  abundance.  The  principle  of  diffusion  is 
operative  in  so  many  of  the  physiological  processes  of  plants 
that  it  must  be  thoroughly  grasped  if  these  processes  are  to  be 
understood. 

When  two  liquids  are  separated  by  a  membrane  through  which 
they  can  pass,  diffusion  between  them  will  still  take  place.  Such 
diffusion  through  a  membrane  is  called  osfnosis,  and  it  tends  to 
continue  (if  the  permeability  of  the  membrane  allows)  until  the 
composition  of  the  liquids  on  both  sides  of  the  membrane  is  the 
same.  If  a  solution  of  salt  in  water,  for  example,  is  present 
on  one  side  and  pure  water  on  the  other,  the  salt  will  tend  to 
diffuse  through  the  membrane  until  its  concentration  is  the  same 
throughout.  It  is  important  to  note  that  the  concentration 
(the  amount  of  substance  dissolved  per  unit  of  volume)  rather 
than  the  total  amount  of  the  substance  or  the  bulk  of  the  solution, 
is  the  factor  which  determines  the  direction  and  rate  of  diffusion. 
It  is  by  diffusion  through  the  cytoplasmic  membranes  of  the 
root-hair  that  mineral  salts  in  the  soil  solution  enter  the  plant. 
This  inward  movement  of  any  given  salt  will  continue  so  long  as 
its  concentration  is  greater  in  the  soil  water  than  in  the  sap  of 
the  root-hair. 


THE  ROOT  AND  ITS  FUNCTIONS  40 

Osmotic  Movement  of  Solvents. — This  phciioincnon  of  osmosis 
is  complicated,  however,  bj^  the  fact  that  the  dissolving  liquid 
or  solvent  (water  in  this  case)  as\well  as  the  dissolved  substance 
will  pass  through  the  membrane,  and  by  the  remarkable  circum- 
stance that  where  such  movement  occurs,  it  is  always  more  rapid 
in  one  direction  than  in  the  other.  Experiment  has  shown  that 
if  two  solutions  of  different  densities  (or  a  solution  and  pure 
water)  are  separated  by  a  membrane,  a  movement  of  water  takes 
place  from  the  less  concentrated  to  the  more  concentrated  solution, 
and  tends  to  continue  till  both  are  of  the  same  density;  and  that 
the  rate  of  this  movement  is  proportional  to  the  difference  in 
concentration.  The  more  concentrated  solution  will  therefore 
tend  to  expand  through  this  access  of  water,  and,  if  it  is  confined 
within  a  closed  space,  a  pressure,  often  of  considerable  magni- 
tude, will  develop.* 

As  to  why  such  a  movement  of  water  occurs  no  complete 
agreement  of  opinion  yet  exists,  for  the  process  of  osmotic  inter- 
change involves  some  of  the  less  clearly  understood  phenomena  of 
physical  chemistry.  We  may  assume  that  the  dissolved  sub- 
stance has  an  affinity  or  attractive  power  for  water,  and  that  this 
attraction  increases  with  the  concentration  of  the  substance;  or 
that  the  molecules  of  the  dissolved  substance  interfere  with  the 
free  molecular  movement  of  water  molecules,  so  that  where  there 
is  little  material  in  solution  the  water  molecules  strike  the 
membrane  and  pass  through  it  oftener  than  they  can  where  much 
material  is  in  solution;  or  we  may  regard  the  whole  phenomenon 
as  really  a  manifestation  of  the  fundamental  principle  of  diffusion, 
since  the  tendency  is  for  the  solutions  on  both  sides  of  the  mem- 
brane to  become  equal  in  concentration,  although  this  is  here 
brought  about  by  a  movement  of  the  dissolving  liquid  as  well  as 
of  the  dissolved  substance  itself.  None  of  these  explanations  is 
entirely  satisfactory,  but  they  may  perhaps  help  to  picture  the 
process  more  clearly  to  our  minds.  The  essential  fact  remains 
that  water  will  pass  through  a  membrane  toward  the  denser 
solution,  explain  it  as  we  may;  and  upon  this  fact  depends  the 
power  of  the  plant  to  withdraw  water  from  the  soil. 

Permeability  of  the  Membrane. — Thus  far,  we  have  assumed 
that  both  the  dissolved  substance  and  water  may  pass  with  per- 

*  Osmotic  phenomena  arc  shown  by  other  solvents  than  water,  but  since 
water  is  the  all-important  solvent  in  physiology,  we  shall  confine  our  at- 
tention only  to  those  osmotic  processes  in  which  it  is  involved. 


50  BOTANY:  PRINCIPLES  AND  PROBLEMS 

feet  freedom  through  the  iiieinl3rane,  or  that  the  meinbi'ane  is 
permeable  to  them.  All  osmotic  membranes  are  readily  perme- 
able to  water,  but  we  find  that  they  differ  markedly  in  the  ease 
with  which  dissolved  substances  of  various  sorts  can  diffuse 
through  them.  One  membrane  may  be  perfectly  permeable  to  a 
given  substance;  another  may  allow  it  to  pass  slowly  and  with 
difficulty,  and  another  may  exclude  it  altogether.  Nor  does  even 
the  same  membrane  display  an  equal  degree  of  permeability  to 
all  substances,  for  some  may  pass  through  it  easily,  others  with 
difficulty,  and  others  not  at  all.  To  what  these  differences  in 
permeability  are  due  we  do  not  know,  but  they  are  presumably 
caused  by  the  relations  between  the  structure  of  the  membrane 
and  the  size  and  character  of  the  molecules  of  the  dissolved 
substance. 

A  membrane  which  allows  water  to  diffuse  through  it  but  does 
not  allow  a  given  dissolved  substance  to  do  so  is  called  semi- 
permeable, and  it  is  a  highly  important  biological  fact  that  all 
membranes  in  living  cells  seem  to  belong  to  this  class.  The 
membrane  of  a  root-hair  cell,  for  example,  allows  water  to  pass 
readily  but  is  impermeable  to  such  substances  as  sugar,  which  are 
dissolved  in  the  sap  solution.  The  cell  is  thus  able  not  only  to 
retain  these  valuable  materials  within  itself,  unwasted  by  outward 
diffusion,  but  to  use  them  as  a  permanent  means  of  drawing  in 
osmotically  a  supply  of  water  from  the  soil,  since  their  presence 
within  the  root-hairs  normally  maintains  the  sap  of  these  cells  at 
a  higher  concentration  than  the  adjacent  soil  solution.  This 
same  membrane,  however,  is  permeable  to  most  of  the  mineral 
salts  present  in  the  soil,  which  are  thus  able  to  diffuse  readily 
into  the  root-hair. 

Other  Principles  of  Osmotic  Action. — Before  we  attempt  to 
apply  the  principles  of  osmosis  to  the  living  plant,  however,  we 
should  fix  clearly  in  mind  certain  facts  with  regard  to  osmotic 
phenomena  in  general  about  which  confusion  frequently  arises. 
First,  substances  which  are  not  soluble  or  which  for  any  reason 
are  not  in  solution  cannot  diffuse  and  have  no  osmotic  effect 
whatever.  Sugar,  for  instance,  is  soluble  and  is  osmotically 
active,  but  the  moment  it  is  converted  into  starch,  which  is  an 
insoluble  substance,  it  loses  its  osmotic  effect  entirely.  Second, 
the  osmotic  strength  of  a  solution,  and  consequently  its  power  to 
attract  water,  is  determined  not  by  the  chemical  nature  of  the 
dissolved  substances  but  by  the  total  concentration  of  material, 


THE  ROOT  AND  ITS  FUNCTIONS  51 

of  whatever  kind,  which  is  in  solution.  A  solution  of  sugar,  one 
of  salt,  one  of  a  mixture  of  the  two,  or  one  containing  half  a  dozen 
substances,  may  all  have  exactly  the  same  osmotic  concentration. 
Third,  the  diffusion  of  water  through  a  membrane  and  the  diffu- 
sion of  salts  through  the  same  membrane  occur  quite  independently 
of  one  another.  Water  will  move  through  a  membrane  from  a 
solution  of  lesser  to  a  solution  of  greater  total  concentration,  but 
a  dissolved  substance,  following  the  general  law  of  diffusion,  will 
pass  from  a  point  where  that  substance  is  abundant  to  one  where 
it  is  scarce,  always  providing  that  the  membrane  is  permeable 
to  it.  Given  the  proper  conditions,  it  is  quite  possible  for  a 
dissolved  substance  to  pass  through  a  membrane  osmotically 
with  no  movement  of  water  taking  place  at  all,  or  for  water  to 
move  without  a  movement  of  dissolved  substances,  or  even  for 
water  to  pass  in  one  direction  and  dissolved  substances  in  the 
other.  Fourth,  if  there  is  more  than  one  substance  in  solution, 
each  will  tend  to  diffuse  quite  independently  of  all  others. 
Differences  in  the  concentration  of  each  substance,  considered  by 
itself,  are  what  determine  the  rate  and  direction  of  diffusion  of 
that  substance. 

Diffusion  and  Osmosis  in  the  Plant  Cell. — It  is  upon  the 
principles  of  diffusion  and  osmosis  that  the  plant  depends,  not 
only  for  the  absorption  of  water  and  mineral  substances  from 
the  soil,  but  for  most  of  the  circulation  of  materials  which  goes 
on  within  the  plant  body.  We  have  already  outlined  briefly  the 
structure  of  a  typical  plant  cell  and  may  now  consider  the 
osmotic  interchanges  which  go  on  therein. 

The  cell  wall  in  plants  is  ordinarily  composed  of  cellulose. 
Like  most  organic  materials,  cellulose  has  the  capacity  of 
absorbing  water  vigorously  by  imbibition  and  will  therefore 
swell  considerably  if  placed,  when  dry,  in  contact  with  water. 
This  expansive  ability  of  the  cell  wall  is  of  some  value  in  certain 
of  the  plant's  activities,  as  in  the  germination  of  the  seed,  but 
the  wall  of  an  ordinary  living  cell  is  moist  and  has  imbibed 
water  to  the  limit  of  its  capacity.  Water,  and  practically  all 
substances  in  solution,  pass  through  this  cellulose  wall  with  great 
readiness,  and  since  it  thus  offers  practically  no  resistance  to 
diffusion,  its  osmotic  effect  is  slight. 

We  have  noted  that,  in  the  mature  plant  cell,  the  cytoplasm 
is  dispersed  in  a  thin  layer  closel}^  pressed  against  the  inner  surface 
of  the  cell-wall,  and  that  it  completely  surrounds  a  large  central 


52  BOTANY:  PRINCIPLES  AND  PROBLEMS 

vacuole  or  sap-cavity,  filled  with  water  in  which  various  sub- 
stances, sugar  usually  prominent  among  them,  are  dissolved.  On 
its  outer  surface  next  the  wall,  and  on  its  inner  surface  next  the 
sap-cavity,  the  cytoplasm  is  bounded  by  a  delicate  membrane,  so 
that  we  find  here  fulfilled  all  conditions  necessary  for  osmotic 
activity — one  solution,  in  the  sap-cavity,  separated  by  a  mem- 
brane or  membranes  from  another  solution,  which  may  be  the 
soil  water  (in  the  case  of  a  root-hair)  or  the  sap-solution  of  an 
adjacent  cell. 

These  cytoplasmic  membranes,  unlike  the  cell-wall,  offer 
resistance  to  the  diffusion  of  certain  things  and  are  thus  highly 
important  in  cell  physiology.  We  find  that  they  are  character- 
istically semi-permeable,  preventing  the  passage  of  such  sub- 
stances as  sugar,  which  are  dissolved  in  the  sap-cavity ;  and  we  have 
already  noted  that  the  cell  is  thus  able  to  retain  these  valuable 
materials  within  itself  and  to  use  them  as  a  means  for  bringing 
in  osmotically  a  continuous  supply  of  water  from  the  soil  or  from 
adjacent  cells.  To  the  essential  mineral  salts  and  to  many 
other  dissolved  substances,  however,  these  membranes  are 
generally  permeable,  though  in  varying  degrees,  and  the  cell  is 
therefore  readily  able  to  absorb  a  supply  of  such  substances  from 
any  adjacent  solution.  It  has  been  found  by  experiment  that 
the  degree  of  permeability  of  the  cell  membranes  is  not  a  fixed 
and  constant  one  but  is  subject  to  change  from  moment  to 
moment  in  response  to  changes  in  the  environment  or  in  the 
protoplasm  itself.  A  cell  which  at  one  time  admits  a  given 
substance  very  readily  may  at  another  allow  it  to  enter  but 
slowly,  or  may  exclude  it  altogether.  Many  of  the  physio- 
logical activities  of  the  cell  are  probably  regulated  by  changes 
in  the  permeability  of  its  membranes. 

The  rapidity  with  which  a  substance  passes  through  a  mem- 
brane is  due  not  only  to  these  differences  in  permeability  but  to 
differences  in  the  concentration  of  the  solutions  on  the  two  sides 
of  the  membrane.  Where  this  difference  is  great  (other  things 
being  equal)  osmotic  diffusion  will  be  more  rapid  than  where  it 
is  slight.  Therefore  if  the  concentration  of  a  dissolved  substance 
within  a  cell  is  reduced,  either  by  its  diffusion  into  an  adjacent 
cell  or  its  conversion  into  an  insoluble  form  (as  must  occur  when 
it  enters  into  the  construction  of  a  complex  organic  molecule  in 
the  protoplasm)  the  rate  at  which  a  new  supply  enters  the  cell 
from   without  is  at   once   correspondingly   increased;   but  the 


THE  ROOT  AND  ITS  FUNCTIONS 


53 


moment  its  concentration  becomes  equal,  within  and  without 
the  cell,  the  movement  of  this  particular  substance  ceases,  even 
though  others  are  passing  rapidly  through  the  membranes. 

The  Absorption  of  Water  and  Salts. — This  activity  of  the  cell 
as  an  osmotic  sj'stcm  evidently  controls  its  most  important 
functions.  Let  us  first  consider  the  role  played  by  osmosis  in 
that  process  which  is  the  immediate  subject  of  this  chapter,  the 


Fig.  30. — Movement  of  water  and  dissolved  substances  into  the  root.  Dia- 
gram showing  the  entrance  of  water  and  dissolved  salts  into  two  root-hairs  and 
their  passage  thence  through  the  cortical  cells  of  the  root.  The  cytoplasmic 
membranes  are  readily  permeable  to  water  and  salts  but  prevent  the  passage  of 
sugar  dissolved  in  the  cell-sap. 


absorption  of  water  and  nutrient  materials  from  the  soil  (Fig. 
30).  Each  root-hair,  as  we  have  seen,  is  merely  a  projection 
from  one  of  the  epidermal  cells  of  the  root.  The  cytoplasm 
and  sap-cavity  of  the  cell  extend  into  the  root-hair,  the  whole 
of  which  is  thus  lined  by  a  thin  cytoplasmic  layer,  Avith  its 
membranes  (Fig.  25).  The  root-hair  penetrates  the  soil  and 
comes  intimately  in  contact  with  the  soil-particles,  to  the  sur- 
face of  each  of  which  a  thin  water-film  normally  adheres.  In  this 
water  are  dissolved  a  great  variety  of  substances,  but  the  total 
concentration  of  the  soil  solution  is  normally  less  than  that  in  the 


54  BOTANY:  PRINCIPLES  AND  PROBLEMS 

sap-cavity  of  the  root-hair,  where  sugar  and  other  materials  are 
dissolved.  In  obedience  to  the  law  of  osmosis,  therefore,  water 
will  pass  from  the  soil  solution  through  the  cytoplasmic  mem- 
branes of  the  root-hair  and  into  its  sap-cavity.  This  flow  of  water 
will  continue  so  long  as  there  is  a  difference  in  total  density 
between  soil-solution  and  sap-solution.  Of  course  if  the  soil 
becomes  dry  and  the  film  around  each  particle  grows  so  thin  that 
the  surface  attraction  of  the  particle  equals  the  osmotic  attrac- 
tion of  the  root-hair  solution,  the  flow  will  necessarily  cease;  and 
if  this  condition  occurs  throughout  the  whole  soil  mass,  the  plant 
will  suffer  from  drought. 

Salts  and  other  substances  in  the  soil-solution  diffuse  through 
the  root-hair  membranes  quite  independently  of  the  passage  of 
water,  and  the  rate  at  which  they  enter  depends  upon  the  factors 
which  we  have  above  considered.  Any  substance  which  is  in 
greater  concentration  in  the  cell-sap  than  in  the  soil-water,  and  to 
which  the  membranes  are  permeable,  will  of  course,  diffuse 
outwardly  into  the  soil;  but  except  for  carbon  dioxide,  which  is 
given  off  in  considerable  quantities  as  a  product  of  respiration  in 
the  root,  there  seems  to  be  comparatively  little  loss  of  material 
from  the  plant  in  this  manner. 

As  water  is  taken  into  the  sap-cavity  of  the  root-hair,  the 
solution  there  becomes  less  dense ;  and  the  first  cell  of  the  cortex 
is  consequently  able  in  turn  to  withdraw  water  osmotically  from 
the  root-hair  cell.  The  second  row  of  cortical  cells  may  now 
withdraw  water  from  the  first,  and  this  process  will  continue  until 
the  water  reaches  the  central  cylinder.  The  water-ducts  here, 
however,  are  nothing  but  dead  shells,  their  living  cytoplasm 
having  disappeared  as  soon  as  the  thick  cell  walls  were  completed. 
They  are  filled  with  water,  and  it  is  hard  to  understand  why 
water  should  move  into  them  from  the  cells  of  the  cortex  rather 
than  in  the  reverse  direction.  In  the  innermost  layer  of  cortical 
cells,  a  considerable  pressure  is  probably  developed  by  osmosis, 
and  water  may  simply  be  squeezed  through  the  cytoplasm  and 
into  these  ducts.  We  know,  at  any  rate,  that  water  is  forced  up 
through  the  ducts  under  a  good  deal  of  pressure.  This  root- 
pressure  may  be  measured  by  a  gauge  attached  to  an  opening  in 
the  stem.  As  to  what  causes  water  to  rise  to  great  heights  in 
the  trunks  of  trees  we  shall  speak  later;  but  root-pressure  is 
apparently  only  one  of  the  factors  involved. 


THE  ROOT  AND  ITS  FUNCTIONS  55 

Other  Osmotic  Phenomena  in  the  Plant. — Not  only  the 
absorption  of  water  from  the  soil,  but  the  whole  process  of  circula- 
tion within  the  plant  body,  as  well,  is  primarily  an  osmotic  one; 
for  the  salts  taken  in  by  the  root-hairs,  and  any  dissolved  sub- 
stances in  other  cells  throughout  the  plant,  move  from  cell  to 
cell  by  diffusion  through  the  cytoplasmic  membranes. 

Still  another  contribution  of  osmosis  to  the  plant's  activities 
lies  in  maintaining  the  turgidity  of  the  tissues.  It  is  evident 
that  if  a  cell  has  a  strong  sap-solution  and  is  thus  able  to  absorb 
water  vigorously,  it  will  become  plump  and  fully  expanded  and  will 
press  tightly  against  its  neighbors.  If  all  the  cells  become 
turgid  in  this  way  the  whole  plant  will  tend  to  be  erect  and  rigid, 
like  an  inflated  balloon.  In  the  case  of  parts  which  do  not  possess 
a  firm  skeleton,  such  as  the  leaf  blades,  floral  organs,  or  other 
comparatively  soft  structures,  this  turgidity  is  necessary  to  main- 
tain their  form,  firmness,  and  proper  functioning.  Conversely, 
if  a  cell  is  exposed  to  a  solution  of  greater  concentration  than 
its  cell-sap,  water  will  be  withdrawn  from  it,  it  will  collapse, 
and  its  cytoplasm  will  be  pulled  away  from  the  walls.  Such  a 
condition  of  plasmolysis,  if  extreme  or  long-continued,  will  result 
in  the  death  of  the  cell;  and,  if  extensive,  in  the  death  of  the  plant. 

Osmosis  also  plays  an  essential  part  in  growth,  for  at  any 
growing  region  we  find  a  point  where  the  cells  are  multiplying  in 
number  but  are  still  small,  and  another  point  behind  this  where 
each  expands  rapidly  to  its  final  size.  This  expansion,  with  the 
consequent  stretching  of  the  cell-walls  and  growth  of  the  tissues, 
is  due  to  the  rapid  absorption  of  water  by  the  young  and  delicate 
cells,  the  sap  of  which  is  rich  in  dissolved  sugar.  The  force 
exerted  by  any  growing  region  is  thus  primarily  due  to  osmotic 
pressure. 

Other  Functions  of  the  Root. — We  have  briefly  discussed  the 
root  as  an  organ  of  anchorage  and  of  storage,  and  in  more  detail 
as  an  organ  of  absorption.  It  has  less  frequently  certain  other 
functions  which  should  be  mentioned  here.  Roots  may  arise 
from  almost  any  part  of  the  stem  and  sometimes  from  the  leaves. 
In  many  climbing  plants  they  are  produced  abundantly  on  these 
aerial  organs  and  serve  to  hold  the  plant  firmly  to  its  support. 
In  corn,  stout  roots  arise  from  the  stem  at  a  little  distance  above 
the  ground  and  pass  into  the  soil,  thus  acting  as  props  or  guy- 
ropes  for  the  tall  plant.  In  epiphytes,  the  roots  are  sent  out 
directly  into  the  air  and  possess  a  characteristic  spongy  envelope 


56  BOTANY:  PRINCIPLES  AND  PROBLEMS 

which  absorbs  and  holds  rain-water  and  dew.  In  parasitic  plants 
the  roots  are  converted  into  short,  sucking  organs,  penetra- 
ting the  host-plant  and  withdrawing  therefrom  the  food  upon 
which  the  parasite  lives. 

The  root  and  the  leaf  are  the  two  most  important  vegetative 
organs  of  the  plant,  and  it  is  therefore  the  leaf  which  we  shall 
next  discuss. 


QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

124.  Which  is  apt  to  be  more  regular  and  symmetrical  in  shape, 
the  root-system  or  the  stem-system  of  a  plant?     Why? 

125.  In  a  young  plant  growing  from  the  seed,  the  root  is  larger  and 
better  developed  at  first  than  the  stem  or  leaves.     Explain. 

126.  Tap-roots  are  usually  tapering  in  shape,  being  broadest  near  the 
surface  of  the  ground  and  gradually  narrowing  below.     Explain. 

127.  Do  all  roots  grow  directly  downward,  provided  that  there  are 
no  obstacles  in  the  way?     Give  evidence  for  your  answer. 

128.  It  has  been  found  that  for  most  species  of  plants  there  is  a  rather 
definite  distance  below  the  surf  ace.  of  the  ground  at  which  the  bulk  of 
the  roots  are  formed.  Some  species  are  deep-rooted,  some  shallow,  and 
others  intermediate.  What  physiological  differences  between  the  plants 
can  you  suggest  which  might  cause  these  structural  differences? 

129.  Why  does  the  planting  of  grass  on  sand-dunes  stop  the  drifting 
of  the  sand? 

130.  Why  does  a  covering  of  grass  turf  or  other  close  plant  growth 
prevent  the  erosion  of  soil  on  steep  banks  and  other  exposed  situations? 

131.  As  a  "soil-binder"  to  prevent  erosion  would  a  crop  of  carrots  or  a 
cover  of  grass  be  better?     Why? 

132.  If  from  around  a  tree  which  has  been  growing  in  the  forest  all 
its  neighboring  trees  are  cut  down,  it  is  then  much  more  likely  to  blow 
over  than  is  a  tree  which  has  always  grown  in  the  open.     Why? 

133.  If  a  tree-trunk  is  partly  buried,  for  instance  through  raising  the 
level  of  the  soil  about  it  by  grading  or  other  means,  why  is  the  tree  likely 
to  die? 

134.  Which  type  of  plant  do  you  think  would  withstand  a  drought 
better,  one  with  a  fibrous  root  or  one  with  a  tap-root?     Why? 


THE  ROOT  AND  ITS  FUNCTIONS  57 

135.  Whicli  do  you  think  will  tend  to  have  the  deeper  root-system, 
a  bog  plant  or  a  desert  plant?     Why? 

136.  Do  you  think  that  a  root-systeui  will  tend  to  spread  further  in 
rich  soil  or  in  poor  soil?     Why? 

137.  Roots  will  usually  grow  toward  a  supply  of  nutrient  material  in 
the  soil.     Can  you  suggest  what  causes  them  to  do  this? 

138.  Root-hairs  are  entirely  absent  on  the  older  portions  of  a  root. 
Why? 

139.  Root-hairs  are  absent  from  the  growing  region  at  the  tip  of  the 
root.     Of  what  advantage  is  this  fact  to  the  plant? 

140.  Of  what  advantage  are  root-hairs  to  the  plant  aside  from  their 
function  in  the  absorption  of  water  and  salts? 

141.  Root-hairs  are  commonly  absent  in  water  plants.     Explain. 

142.  The  fibro-vascular  system  of  the  root  is  usually  in  the  form  of  a 
solid  rod,  and  that  of  the  stem  in  the  form  of  a  hollow  tube.  Of  what 
advantage  to  the  plant  are  these  arrangements  of  the  tissues? 

143.  Why  is  the  soil  immediately  around  the  base  of  a  tree  trunk 
somewhat  higher  than  the  adjacent  soil? 

144.  Give  an  example  (other  than  those  cited  in  the  text)  of  a  plant 
which  has  a  fibrous  root-system;  a  tap-root;  climbing  roots;  parasitic 
roots. 

145.  Why  is  protoplasm  regarded  as  the  only  really  "living"  sub- 
stance in  the  plant? 

146.  What  are  some  of  the  ad\-antages  of  cellular  structure  in  plants 
and  animals? 

147.  Give  all  the  reasons  you  can  think  of  for  the  fact  that  cells 
should  be  such  very  small  objects. 

148.  Why  do  so  many  cells  have  a  six-sided  appearance? 

149.  Of  what  advantage  is  the  streaming  of  protoplasm  which  com- 
monly takes  place  in  the  cell? 

150.  What  important  functions  does  the  cell- wall  perform? 

151.  Are  there  any  cells  of  the  plant  which  are  useful  to  the  plant 
after  they  die? 

152.  If  a  sac  made  of  bladder  or  a  similar  osmotic  membrane  is  filled 
with  molasses,  tied  up,  and  placed  in  a  vessel  of  water,  what  will  hai)pen? 


58  BOTANY:  PRINCIPLES  AND  PROBLEMS 

153.  If  this  same  sac  is  filled  with  water,  tied  up  and  placed  in  a 
vessel  of  molasses,  what  will  happen? 

154.  Under  what  conditions  will  water  pass  through  a  membrane 
osmotically  without  any  movement  of  dissolved  substances  taking  place? 
When  would  this  be  likely  to  happen  in  the  root-hairs  of  plants? 
Explain. 

155.  Under  what  conditions  will  a  dissolved  substance  pass  through 
a  membrane  without  any  movement  of  water  taking  place?  When 
would  this  be  likely  to  happen  in  the  root-hairs  of  plants?     Explain. 

156.  Under  what  conditions  will  water  pass  through  a  membrane  in 
one  direction  and  a  dissolved  substance  in  the  other?  When  would 
this  be  likely  to  happen  in  the  root-hairs  of  plants?     Explain. 

Note. — Assume  in  the  five  following  questions  that  the  sac  is  made  of 
an  elastic  membrane  which  is  semi-permeable  in  that  it  allows  the  free 
diffusion  of  water  through  it  but  entirely  prevents  the  passage  of  sugar. 
Assume  further  that  this  membrane  is  readily  permeable  to  dissolved 
salt.  Under  the  conditions  set  forth  in  each  question,  state  clearly  what 
will  happen,  carrying  the  process  through  to  its  conclusion  and  noting 
the  differences,  if  any,  between  early  and  later  stages. 

157.  What  will  happen  if  the  sac  is  filled  with  a  solution  of  sugar  and 
placed  in  a  vessel  of  pure  water? 

158.  What  will  happen  if  the  sac  is  filled  with  a  solution  of  sugar  and 
the  liquid  in  the  vessel  is  a  solution  of  salt  which  is  of  lesser  concentration 
than  the  sugar  solution  in  the  sac? 

159.  What  will  happen  if  the  sac  is  filled  with  a  solution  of  sugar  and 
the  liquid  in  the  vessel  is  a  solution  of  salt  which  is  equal  in  concentra- 
tion to  the  sugar  solution  in  the  sac? 

160.  What  will  happen  if  the  sac  is  filled  with  a  solution  of  sugar  and 
the  liquid  in  the  vessel  is  a  solution  of  salt  which  is  greater  in  concentra- 
tion than  the  sugar  solution  in  the  sac? 

161.  What  will  happen  if  the  sac  is  filled  with  a  mixture  of  sugar  and 
salt  solutions  of  equal  concentration  and  that  the  hquid  in  the  vessel 
is  pure  water? 

162.  Can  a  plant  take  out  all  the  water  from  the  soil  around  its  roots? 
Explain. 

163.  The  stems  of  most  woody  plants  will  "bleed"  if  cut  in  the 
spring,  but  will  not  do  so  if  cut  in  the  summer.     Explain. 

164.  Why  do  tomatoes  and  other  soft  and  fleshy  fruits  tend  to  split 
open  in  a  wet  season? 


THE  ROOT  AND  ITS  FUNCTIONS  59 

165.  How  does  it  happen  that  a  plant  can  take  up  such  large  amounts 
of  salts  when  these  salts  occur  in  such  very  weak  concentrations  in  the 
soil? 

166.  Many  plants  thrive  for  long  periods  without  extending  their 
root-systems  into  fresh  regions  of  soil.  How  are  they  able  to  obtain  an 
unfailing  supply  of  nutrient  salts  under  these  conditions? 

167.  A  plant  grown  in  "water-culture"  (in  a  jar  of  water  containing 
the  necessary  nutrient  salts  in  solution)  will  almost  completely  remove 
these  salts  from  the  jar,  even  though  its  roots  fill  only  a  small  part  of  the 
jar.     Explain. 

168.  Iodine  is  much  more  abundant  in  the  tissues  of  certain  sea- 
weeds than  it  is  in  the  sea  water.     Explain  how  this  can  be. 

169.  How  is  it  possible  for  a  group  of  cells  in  the  middle  of  a  tissue, 
surrounded  by  other  cells,  to  contain  large  amounts  of  a  substance  which 
is  rare  or  absent  in  the  other  cells? 

170.  The  text  states  that  the  cell-membrane  of  a  root-hair  is  imperme- 
able to  sugar,  and  that  sugar  therefore  cannot  get  out  of  the  root-hair 
into  the  soil.  If  this  is  true,  how  do  you  think  the  sugar  was  able  to  get 
into  the  root-hair  in  the  first  place? 

171.  A  crop  plant  which  removes  a  large  amount  of  nutrient  material 
from  the  soil  is  known  as  a  "heavy  feeder"  and  one  which  removes 
little  as  a  "light  feeder."  What  factors  can  you  think  of  which  would 
cause  plants  to  differ  in  this  respect? 

172.  The  salts  taken  from  the  soil  by  one  plant  are  often  very  differ- 
ent in  kind  and  amount  from  those  taken  in  by  another  plant.  To  what 
factors  may  these  differences  be  due? 

173.  One  crop  often  needs  a  different  fertiUzer  from  another.  To 
what  physiological  differences  in  the  two  crop-plants  may  this  be  due? 

174.  How  do  submersed  water-plants  get  their  salts? 

175.  Some  fertilizers,  when  applied  very  abundantly,  will  often  kill 
plants.     Why? 

176.  A  spray-solution  which  is  strongly  concentrated  will  often  kill 
plants  to  which  it  is  applied.     Why? 

177.  A  spray-solution  which  will  kill  one  plant  may  not  kill  another. 
Explain. 

178.  Strong  spray  will  often  kill  the  young  and  growing  parts  of  a 
plant  but  not  the  older  portions.     Why? 


60  BOTANY:  PRINCIPLES  AND  PROBLEMS 

179.  If  a  very  strong  spray  (such  as  lime-sulphur,  used  against  cer- 
tain bark-insects  of  fruit  trees)  is  to  be  applied  to  a  tree,  why  must  this 
be  done  only  wlien  the  tree  is  leafless? 

180.  In  such  places  as  gravel  walks  and  tennis  courts  it  is  often 
customary  to  kill  weeds  by  sprinkling  salt  upon  them.  Why  is  this 
practice  effective?     Why  is  it  not  widely  used  in  killing  farm  weeds? 

181.  How  is  it  possible  for  some  plants  to  live  on  salt-marshes  and 
sea-beaches  while  others  cannot? 

182.  Desert  plants  and  salt-marsh  or  sea-beach  plants  frequently 
show  similar  modifications  in  structure.     Explain. 

183.  Why  do  salt-marsh  or  sea-beach  plants  usually  die  if  subjected 
to  fresh  water? 

184.  The  sap  concentration  in  the  cells  of  parasitic  plants  has  been 
found  to  be  higher  than  in  the  cells  of  the  plants  upon  which  they  are 
parasitic.     Explain. 

185.  Why  do  dried  currants,  raisins,  and  prunes  swell  so  much  when 
placed  in  water? 

186.  When  berries  are  cooked  with  little  sugar  they  are  apt  to  burst. 
When  cooked  with  much  sugar  they  are  apt  to  collapse.     Explain. 

187.  Vegetables  usually  cook  more  quickly  if  they  are  not  salted 
while  cooking.     Explain. 

188.  Celery,  sliced  cucumbers  and  similar  vegetables  are  often 
placed  in  water  for  a  while  before  they  are  served.  What  effect  does 
this  produce  and  why? 

189.  Why  are  we  thirsty  after  eating  much  salt  or  sugar? 

Note. — In  the  five  following  questions,  remember  that  decay  is  due 
to  the  activity  of  bacteria,  which  are  merely  very  small  plant  cells 
(p.  285). 

190.  Why  is  salt  sugh  a  good  perservative  of  vegetables,  meat,  fish, 
and  other  foods? 

'     191.  Which  will  "keep"  better  if  exposed  freely  to  the  air,  grape 
juice  or  grape  jelly?     Why? 

192.  Old-fashioned  preserving  of  fruit  was  done  by  the  "pound  for 
pound"  method,  a  pound  of  sugar  being  used  for  every  pound  of  fruit. 
Why  was  this  method  successful  even  in  the  absence  of  boiling  or  any 
other  means  of  sterilization? 


THE  ROOT  AND  ITS  FUNCTIONS  61 

193.  What  is  the  fundamental  difference  between  preserving  food  bj^ 
salt  and  preserving  it  b}^  benzoate  of  soda  or  a  similar  substance? 

194.  A  little  salt  placed  in  the  water  in  which  cut  flowers  are  standing 
will  often  cause  them  to  keep  fresh  longer  than  they  otherwise  would. . 
Explain.  Would  a  large  amount  of  salt  in  the  water  have  the  same 
result?     Explain. 

195.  What  causes  the  root-hairs  to  force  themselves  in  among  the 
soil  particles? 

196.  Growing  roots  and  stems  often  exert  tremendous  pressures, 
sometimes  sufficient  to  split  and  lift  heavy  rocks.  What  causes  this 
expansive  power? 

197.  Rocks  are  sometimes  split  apart,  in  quarrying,  by  the  insertion 
of  dry  wooden  wedges  into  drills  or  cracks.  These  wedges  are  then 
wetted,  and  their  swelling  exerts  a  powerful  pressure.  How  different 
in  its  origin  is  this  pressure  from  that  exerted  by  a  tree-root  which  splits 
open  a  rock? 

198.  Rapidly  growing  beets,  turnips,  and  similar  fleshy  plant  parts 
sometimes  crack  open  during  growth.     Why? 

199.  Water  is  sometimes  forced  from  the  leaves  of  a  plant  in  the  form 
of  droplets.  Why  is  this  and  under  what  conditions  is  it  likely  to  take 
place? 

REFERENCE  PROBLEMS 

22.  Are  the  "root-crops"  (such  as  carrots,  parsnips,  turnips,  and  beets) 
usually  annual,  biennial  or  perennial  plants?     Why? 

23.  Are  there  any  fleshy-rooted  plants  which   do  not  have  tap-roots? 

24.  What  important  crop-plant  is  propagated  by  buds  formed  on  its 
roots? 

25.  When  and  by  whom  was  tlie  Cell  Theory  first  formulated? 

26.  When  and  by  wliom  was  the  term  "protoplasm"  in  its  present  sense 
first  used? 

27.  Who  discovered  that  every  cell  has  a  nucleus? 

28.  What  do  we  mean  by  saying  that  protoplasm  is  a  colloidal  substance? 

29.  State  two  fundamental  differences  between  the  typical  plant  cell  and 
the  typical  animal  cell.  With  what  general  differences  between  animals 
and  plants  are  these  cell  differences  connected? 

30.  What  prevents  the  cells  of  a  plant  from  falling  apart? 


62  BOTANY:  PRINCIPLES  AND  PROBLEMS 

31.  Botanists  distinguish  between  physical  drought  and  physiological 
drought.     In  what  does  this  difference  consist? 

32.  What  is  the  chemical  composition  of  the  ash  of  three  important 
crop-plants?  (Take  figures  from  any  reliable  determination.)  How  does  it 
happen  that  the  substances  and  their  proportions  are  different  in  the 
different  crops? 

33.  What  is  the  difference  between  a  nutritive  and  a  balanced  solution  for 
plant  growth? 

34.  What  relation  is  there  between  the  osmotic  pressure  of  a  substance  and 
the  gas  pressure  exerted  by  this  substance  in  its  gaseous  state? 

35.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Protoplasm  Vacuole  Osmosis 

Cytoplasm  Plastid  Imbibition 

Nucleus  Diffusion  Plasmolysis 


CHAPTER  V 

THE  LEAF  AND  ITS  FUNCTIONS 

The  vegetative  organs  of  the  plant  naturally  fall  into  two 
groups:  The  root-system,  situated  in  the  soil  and  concerned 
primarily  with  the  absorption  therefrom  of  water  and  certain 
nutrient  materials;  and  the  stem  and  leaves  (together  called 
the  shoot),  which  unfold  in  the  air  and  are  concerned  primarily 
with  the  manufacture  of  food,  the  raw  materials  for  which  they 


Fig.  31. — Two  types  of  leaf-venation.  At  left,  notted-veincd  leaf  of  Linden. 
A,  blade.  B,  petiole.  C,  stipules.  At  right,  parallel-veined  leaf  of  Solomon's 
Seal. 

derive  from  l)oth  air  and  soil.  Of  the  two  members  of  the  shoot- 
system  the  leaf  is  the  primary  and  more  important  one,  the  stem 
serving  merely  to  expose  the  leaves  to  light  and  air  and  to  pro- 
vide a  means  of  conummication  between  them  and  the  root- 
system.  It  is  logical,  therefore,  for  us  to  follow  our  study  of  the 
root  with  a  sliidy  of  \\\c  leaf. 

The  Structure  of  the  Leaf. — Before  we  can  understand  cleaily 
the  functions  which  the  leaf  pei'forms,  we  shall  need  to  observe 
with  some  care  its  rather  complicated  structure. 

63 


64  BOTANY:  PRINCIPLES  AND  PROBLEMS 

External  Structure  (Figs.  31  and  32). — Externally,  the  typical 
leaf  consists  of  a  broad,  flat,  and  thin  portion,  the  blade,  which 
is  the  seat  of  its  important  activities.  This  is  green  in  color  and 
provided  with  a  system  of  ribs  or  vei7is  of  stouter  texture  than  the 
rest  of  the  tissue.  The  blade  may  sometimes  be  attached  directly 
to  the  stem,  but  is  usually  supported  by  a  leaf-stalk  or  petiole, 
which  holds  it  out  in  a  place  favorable  for  the  performance  of 


Fig.  32. — Simple  and  compound  leaves.     A,  compound  leaf  of  Mountain  Ash, 
with  eleven  leaflets.     B,  simple  leaf  of  Apple. 

its  functions  and  serves  as  a  highway  for  transportation  of  water 
and  food  between  blade  and  stem.  At  the  base  of  the  petiole 
are  often  two  small  appendages,  the  stipules,  the  function  of 
which  is  in  many  cases  obscure. 

Leaves  vary  widely  in  size,  shape,  texture,  margin,  venation, 
and  other  characters.  In  its  outline  the  blade  may  be  even,  or 
it  may  be  lobed  or  sometimes  actually  divided  into  separate 
portions,  the  leaflets,  in  which  state  the  leaf  is  said  to  be  com- 
pound (Fig.  32).     The  margin  is  sometimes  quite  smooth,  but 


THE  LEAF  AND  ITS  FUNCTIONS 


65 


is  more  commonly  broken  into  teeth  of  various  sizes.  The 
vein-system  (Fig.  31),  is  either  parallel  where  the  veins  run  side 
by  side  with  no  conspicuous  branches;  or  netted,  where  they  divide 
and  anastomose  repeatedly.  The  petiole  and  stipules  vary 
greatly  in  their  development. 

Internal  Structure  (Fig.  33). — Internally,  the  structure  of  the 
leaf  is  highly  differentiated.  A  transverse  section  cut  at  right 
angles  to  the  surface  of  the  blade  (Fig.  34)  displays  three  impor- 
tant tissues :  The  epidermis,  or  protective  covering ;  the  mesophyll, 


Fig.  33. — A  small  piece  of  a  typical  leaf-blade,  seen  in  three  planes  and  highly 
magnified.  A,  upper  epidermis,  covered  by  the  cuticle  (in  black).  B,  palisade 
layer.  C,  spongy  layer.  D,  lower  epidermis,  covered  by  cuticle.  E,  stoma  (in 
one  case  seen  in  section).      F,  air  space.      G,  vein,  cut  lengthwise. 


constituting  the  major  portion  of  the  leaf  substance,  and  the 
veiiis,  each  of  which  is  a  separate  fibro-vascular  bundle  and  repre- 
sents a  final  branch  of  the  vascular  system  which  runs  through 
root  and  stem. 

The  epidermis  covers  the  entire  leaf  surface  and  is  usually 
but  one  cell-layer  in  thickness.  Its  cells  are  generally  thin- 
walled  and  are  filled  with  a  transparent  cell-sap.  Spread  over 
the  outside  wall  is  a  thin,  waxy,  water-proofing  layer,  the  cuticle, 
extending  from  cell  to  cell  and  forming  a  continuous  skin  which 
covers  the  epidermis.  It  is  usually  much  thicker  on  the  upper 
than  on  the  lower  surface  of  the  leaf.  The  epidermis  is  not  an 
unbroken  layer  but  is  provided  with  minute  openings,  the 
stoniata  (singular,  stoma),  through  which  an  exchange  of  gases 
between  the  tissues  of  the  leaf  and  the  outside  air  may  take 


66 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


place  (Fig.  35).  These  are  much  more  numerous  in  the  lower 
epidermis  than  in  the  upper,  and,  indeed,  are  often  absent  from 
the  latter  altogether.     Each  stoma  is  a  slit-like  pore  formed  by 


Fig.  34. — Cross  section  through  the  blade  of  a  typical  leaf.  A,  upper  epi- 
dermis, covered  with  cuticle.  B,  lower  epidermis,  also  covered  with  cuticle. 
C,  palisade  layer  of  the  mesophyll.  D,  spongy  layer  of  the  mesophyll.  E, 
stoma.  F,  vein.  (After  Kny.  FromGanong,'"  Textbook  of  Botany" ,  copyrighted 
by  the  Macmillan  Company.     Reprinted  by  permission). 


Fig.  35. — A  stoma.  A,  face  view,  showing  the  two  guard  cells  (containing 
chloroplasts) ;  the  pore  between  them,  and  several  adjacent  cells  of  the  epidermis. 
B,  transverse  section,  with  the  two  guard  cells,  several  adjacent  cells  of  the 
epidermis,  and  a  portion  of  two  palisade  cells  below.  {After  Kny.  From 
Ganong,  "  Textbook  of  Botany",  copyrighted  by  the  Macmillan  Company.  Reprinted 
by  permission) . 

the  pulling  apart  of  two  modified  epidermal  cells,  the  guard- 
cells,  which  are  unlike  other  cells  of  the  epidermis  in  containing 
chlorophyll.     These   guard-cells   are   so   constructed  that  when 


THE  LEAF  AND  ITS  FUNCTIONS  67 

plump  and  turgid  with  water  they  tend  to  pull  apart,  thus 
enlarging  the  opening.  On  becoming  limp  and  partially  collapsed, 
however,  they  spring  together  again  and  close  it.  The  degree 
of  stomatal  opening  is  thus  continually  fluctuating  as  the  water 
supply  of  the  guard-cells  rises  and  falls  in  response  to  changing 
internal  or  external  conditions. 

The  mesophyll  consists  of  tissue  which  is  characteristically 
thin-walled,  soft,  and  green.  The  cytoplasm  within  its  cells 
contains  very  small,  roundish  bodies,  denser  than  the  rest  of  the 
hving  substance,  and  green  in  color.  These  are  the  chloroplasts 
which  contain  within  them  the  green  pigment  chlorophyll,  to 
which  the  characteristic  color  of  foliage  is  due.  The  mesophyll 
is  not  a  homogeneous  tissue  but  in  typical  leaves  is  divided  into 
two  main  regions.  That  part  lying  next  to  the  upper  side 
of  the  leaf  is  composed  of  cells  which  are  elongated  at  right 
angles  to  the  leaf  surface,  packed  rather  closely  together,  and 
provided  with  a  great  abundance  of  chloroplasts  (Fig.  36). 
This  region  is  known  as  the  palisade  layer  and  here  is  carried 
on  most  actively  the  process  of  food-manufacture  or  photo- 
synthesis. The  lower  region  of  the  mesophyll  consists  of  a  mass 
of  cells  which  are  so  very  irregular  in  shape  that  large  air-spaces 
occur  between  them,  and  a  very  loose,  sponge-like  tissue,  the 
spongy  layer,  is  produced.  These  air-spaces  communicate 
directly  with  the  outside  air  through  the  stomata.  Chloro- 
plasts are  present  in  the  spongy  layer,  but  not  abundantly. 
Through  the  exposure  to  the  air  of  a  large  area  of  cell-surface, 
opportunity  is  provided  in  this  portion  of  the  mesophyll  for  those 
gas-exchanges  which  are  continually  taking  place  between  the 
leaf  and  the  atmosphere,  such  as  the  absorption  and  excretion  of 
both  carbon  dioxide  and  oxygen,  in  the  processes  of  photosynthe- 
sis and  respiration,  and  the  evaporation  of  water  in  the  process 
of  transpiration. 

Running  through  the  blade  are  the  fibro-vascular  bundles  or 
veins,  the  channels  by  which  the  leaf  tissues  are  kept  in  communi- 
cation with  the  rest  of  the  plant.  The  main  veins  are  stout, 
often  projecting  somewhat  below  the  lower  surface  of  the  blade. 
These  break  up  into  smaller  and  smaller  veins,  and  finall}^  into 
minute  veinlets  which  consist  of  only  a  few  cells.  Each  vein  is 
surrounded  by  a  bundle-sheath  of  heavy-walled  cells,  to  which 
most  of  its  rigidity  is  due.  Within  this  are  two  tissues:  The 
wood,  consisting  largely  (as  elsewhere  in  the  plant  body)  of  olon- 


68 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


gated,  water-conducting  cells,  the  tracheids  and  duds,  which  dis- 
tribute the  water  and  dissolved  substances  brought  up  through 
the  stem  from  the  root;  and  the  bast,  consisting  of  especially 
modified  cells,  the  sieve-tuhes,  which  collect  from  the  mesophyll 


Chloroplas-f 


Nucleu 


Fig.  36. — A  palisade-cell.  The  chloroplasts  are  somewhat  biscuit-shaped 
bodies,  being  roughly  circular  in  face-view  and  elliptical  when  seen  from  the  side. 
They  are  embedded  in  the  thin  and  transparent  layer  of  cytoi)lasm,  their  broad 
faces  parallel  to  the  cell  wall.  The  chloroplasts  which  here  appear  circular  are 
therefore  lying  against  the  front  wall,  nearest  the  observer.  Those  which  appear 
elliptical  are  lying  next  the  side  walls. 

the  food  manufactured  there  and  convey  it  to  the  bast  of  the 
stem,  along  which  it  is  transported  to  other  parts  of  the 
plant. 

The  petiole,  usually  circular  in  cross  section,  has  within  it  a 
cylinder  or  half-cylinder  of  fibro-vascular  bundles  which  are  con- 


THE  LEAF  AND  ITS  FUNCTIONS  G9 

tiniious  with  the  main  veins  of  the  blade  above  and  which  enter 
directly  into  the  vascular  cylinder  of  the  stem  below. 

Photosynthesis. — The  primary  activity  of  green  leaves  is  the 
manufacture  of  food  from  certain  simple  inorganic  materials — 
carbon  dioxide  and  water — by  energy  derived  from  light.  This 
process  of  pJwtosynthesis  is  fundamental  in  organic  nature,  for  it 
is  not  only  an  essential  function  of  green  plants  themselves  but 
is  of  the  utmost  significance  to  animals  and  man,  because  it 
constitutes  the  sole  ultimate  source  of  food  in  the  world.  Food 
is  primarily  a  storehouse  of  energy  and  of  hody-huilding  materials 
for  living  things.  In  the  green  parts  of  plants,  and  nowhere  else 
among  organisms,  is  active  or  kinetic  energy — in  this  case  the 
energy  of  light — converted  into  a  latent  or  'potential  form,  readily 
available  to  living  organisms  for  use  in  maintaining  their  vital 
activities;  and,  moreover,  in  green  plants  alone  are  produced 
those  fundamental  organic  materials  out  of  which  plant  and  animal 
bodies  are  constructed.  All  the  complex  metabolic  changes  which 
later  take  place  in  the  organic  world  are  simply  elaborations  or 
simplifications  of  the  primary  products  of  photosynthesis.  A 
more  detailed  account  of  the  various  types  of  foods  and  their 
uses,  and  of  the  energy-relations  of  the  plant,  will  be  given  in  our 
chapter  on  Metabolism. 

Materials. — The  materials  combined  by  the  plant  in  this  proc- 
ess are  but  two — water  and  carbon  dioxide.  Water  is  absorbed 
from  the  soil  by  the  roots,  passes  upward  through  the  stem,  the 
petiole,  and  the  veins  of  the  leaf,  and  thence  enters  the  mesophyll 
cells.  None  is  obtained  by  the  leaf  directly  from  the  atmosphere. 
It  should  be  remembered  that  only  a  relatively  small  portion  of 
the  water  taken  in  by  the  plant  is  used  in  food-manufacture;  for 
much  the  larger  part  soon  leaves  the  plant  again,  passing  out  of 
the  leaf  into  the  air  by  transpiration.  The  carbon  dioxide  used 
in  photosynthesis  is  derived  entirely  from  the  air.  Here  it  is 
always  present,  but  in  such  small  quantities  that  it  constitutes 
only  about  three  parts  in  ten  thousand  of  the  atmosphere  or  three 
hundreths  of  1  per  cent.  Experiments  have  shown  that  a  higher 
concentration  would  be  advantageous  to  plant  growth,  since  up  to 
a  certain  point  the  rate  of  photosynthesis  rises  if  the  proportion 
of  carbon  dioxide  in  the  air  is  artificially  increased.  It  is  through 
this  comparativc^ly  rare  gas  alone  that  the  plant  derives  its  supply 
of  carbon,  that  element  so  vitally  necessary  to  all  living  organ- 
isms.    No  other  carbon   compounds,   not  even  the  abundant 


70  BOTANY:  PRINCIPLES  AND  PROBLEMS 

supplies  present  in  the  complex  organic  materials  of  humus,  can 
apparently  be  drawn  upon  by  ordinary  green  plants.  Carbon, 
oxygen  and  hydrogen,  together  with  the  seven  essential  elements 
derived  from  the  soil,  constitute  the  necessary  chemical  basis  for 
plant  life. 

Mechanism. — The  mechanism  or  apparatus  by  which  water  and 
carbon  dioxide  are  combined  is  the  remarkable  green  pigment 
chlorophyll.  This  is  present  only  in  the  chloroplasts,  portions  of 
the  cytoplasm  slightly  denser  than  the  rest.  Chloroplasts  may 
be  few  and  large  in  certain  lower  plants  but  in  the  higher  ones  are 
almost  always  small,  numerous,  and  more  or  less  spherical  in 
shape.  They  are  most  abundant  in  the  palisade  layer  of  the  leaf. 
As  to  chlorophyll  itself  we  know  comparatively  little  except  that 
it  is  a  complex  protein  and  contains  magnesium.  Iron  is  essen- 
tial for  its  production  but  apparently  does  not  enter  into  the 
construction  of  the  substance  itself.  The  presence  of  light  is 
also  necessary  for  the  full  development  of  chlorophyll,  as  is  shown 
by  the  pale  color  of  leaves  which  have  grown  in  darkness.  We  are 
even  more  ignorant  as  to  the  manner  in  which  chlorophjdl  oper- 
ates in  bringing  about  the  union  of  carbon  dioxide  and  water, 
nor  have  we  yet  succeeded  in  imitating  this  process  in  the  labo- 
ratory. We  know,  however,  that  chlorophyll  does  not  con- 
tribute material  to  the  product  formed  and  that  it  is  not  used  up 
itself  in  the  process,  and  we  may  therefore  infer  that  it  acts 
somewhat  as  does  a  catalyzer. 

Associated  with  chlorophyll  is  usually  another  pigment  or 
group  of  pigments,  j^ellow  in  color  instead  of  green,  to  which  the 
general  terms  xanihophyll  or  carotin  are  given.  These  are  not 
concerned  with  photosynthesis  and  their  function  is  poorly 
understood.  To  them  are  due  most  of  the  yellow  colors  which 
occur  in  plants.  Chlorophyll  is  a  very  unstable  compound  and 
tends  to  break  down  quickly  when  extracted  from  the  leaf  or 
when  the  leaf  loses  its  vitality,  but  the  yellow  pigments  are  much 
more  resistant  and  often  survive  long  after  chlorophyll  has 
disintegrated. 

Energy. — Energy  is  necessarily  expended  in  the  process  of 
breaking  up  the  molecules  of  water  and  carbon  dioxide  and  recom- 
bining  their  atoms  into  a  new  compound.  We  know  that  this 
energy  is  derived  not  from  heat,  as  in  so  many  cases,  but  entirely 
from  light,  which  thus  plays  an  essential  part  in  the  physiology 
of  plants.     According  to  the  most  widely  accepted  theory,  light 


THE  LEAF  AXC  ITS  FUNCTIONS 


71 


is  due  to  niinut(>  uiul  (MioihiousIn-  rapid  vilji'aiioiis,  (he  Icii^lh  of 
the  vibration — its  wave-length — dctcnnininp;  the  color  of  the 
Hght  produced.  Sunhght,  or  any  white  hght,  is  composed  of 
vil)rations  of  a  great  variety  of  (Uflerent  wave-lengths,  but  when 
such  light  is  passed  through  a  prism  these  become  sorted  out  into 
a  many-colored  spectrum.  The  visible  spectrum  runs  from  the 
red  rays,  the  wave-length  of  which  is  approximately  750  millionths 
of  a  millimeter,  to  the  violet  ones,  where  it  is  approximately 
400.  These  visible  radiations  are  by  no  means  the  only  ones 
which  occur,  however.     Rays  of  longer  wave-length  than  red — 


P'iG.  37. — Diagrams  of  a  spectroscope,  showing  light  broken  up  by  a  prism  into 
its  constituent  portions,  which  form  a  spectrum.  A,  spectrum  of  white  light. 
B,  spectrum  of  light  which  has  passed  through  a  chlorophyll  solution,  showing  the 
dark  absorption  bands  in  the  red  and  the  blue.  {From  Ganong,  "The  Living 
Plant",  Henry  Holt  and  Co.) 


the  infra-red  rays — pass  gradually  into  heat-waves,  and  those 
shorter  than  violet — the  ultra-violet  rays — are  active  chemically. 
When  falling  upon  different  objects,  light  behaves  differently. 
All  of  it  may  be  absorbed  by  the  object  and  converted  into  heat 
or  some  other  form  of  energy,  the  object  then  appearing  black; 
or  all  may  be  reflected,  the  object  then  appearing  white;  or 
certain  wave-lengths  may  be  absorbed  and  certain  others  reflected, 
the  object  in  such  a  case  displaying  to  our  eyes  the  color  of 


72  BOTANY:  PRINCIPLES  AND  PROBLEMS 

the  light  which  it  reflects.  We  know,  for  example,  that  a  green 
substance  like  chlorophjdl  absorbs  in  general  those  wave-lengths 
which  are  not  green  and  reflects  those  which  are  green  or  greenish- 
yellow.  We  can  determine  more  accurately,  however,  the  kind  of 
light  which  is  absorbed  by  a  substance,  if  we  break  up  into  a 
spectrum  the  light  which  has  passed  through  that  substance. 
Such  a  spectrum  displays  perfectly  dark  regions,  or  absorption 
hands,  in  those  portions  which  correspond  to  the  particular  kinds 
or  wave-lengths  of  light  which  the  substance  has  absorbed.  The 
absorption  spectrum  of  chlorophyll  (Fig.  37)  shows  a  narrow, 
sharp,  black  band  in  the  orange-red  and  a  wider,  less  definite 
one  in  the  blue,  thus  indicating  that  it  is  chiefly  these  two  kinds 
of  light  which  chlorophyll  absorbs,  and  suggesting  that  the  red 
and  blue  rays  in  sunlight,  and  no  others,  furnish  the  energy  used 
in  the  process  of  photosynthesis.  Chlorophyll  possesses  the 
remarkable  power  of  utilizing  energy  from  this  source  in  the 
manufacture  of  food,  an  ability  that  is  unique  in  the  organic 
world. 

The  intensity  as  well  as  the  character  of  the  light  affects  the 
rate  at  which  photosynthesis  proceeds.  The  process  begins  at 
illuminations  of  very  low  intensity,  reaches  its  maximum  at 
about  that  of  bright  diffuse  daylight,  and  decreases  again  in 
light  which  is  so  bright  as  to  injure  protoplasm.  Photosynthesis 
may  be  readily  accomplished  in  artificial  light  of  the  proper 
intensity  and  wave-lengths. 

Given  a  supply  of  the  necessary  raw  materials,  a  sufficient 
temperature,  the  presence  of  chlorophyll,  and  light  of  proper 
character  and  intensity,  photosynthesis  may  go  on  anywhere  in 
the  plant.  Although  these  conditions  are  preeminently  fulfilled 
in  the  mesophyll  of  the  leaves,  they  may  also  be  present  to  a 
lesser  extent  in  petioles,  stipules,  calyx-lobes,  and  other  organs, 
thus  insuring  a  utiHzation  of  these  regions  to  produce  a  small 
supplementary  food  supply. 

Products. — ^Let  us  now  turn  from  a  consideration  of  the  neces- 
sary conditions  for  photosynthesis  to  a  study  of  its  products. 
The  details  of  the  process  whereby  carbon  dioxide  unites  with 
water  are  not  yet  known,  but  the  formation  of  formaldehyde 
(CH2O)  is  perhaps  one  of  the  prehminary  steps.  The  first 
product  which  can  be  recognized,  however,  and  a  substance  which 
is  therefore  of  unique  interest,  is  glucose  or  grape  sugar,  C6H12O6, 
formed  according  to  the  following  equation: 


THE  LEAF  AND  ITS  FUNCTIONS  73 

6CO2  +  6H2O  =  CeHioOe  +  6O2 

Glucose  is  present  in  the  sap  of  practically  all  plant  cells. 
It  is  the  fundamental  carbohydrate  and  the  basis  for  all  other 
foods;  and  from  it  are  ultimately  derived,  through  the  action  of 
enzymes  and  by  various  additions  and  chemical  modifications,  all 
the  organic  compounds  of  plants  and  animals. 

The  presence  of  a  large  amount  of  sugar  in  a  chlorophyll- 
bearing  cell  results  in  a  stoppage  of  its  manufacture  there,  and 
is  disadvantageous  for  other  reasons.  We  find,  accordingly,  that 
before  photosynthesis  has  long  continued,  the  resulting  sugar 
becomes  converted  into  another  type  of  carbohydrate,  starch 
(CeHioOs)!!.*  Starch  is  complex  and  insoluble,  occurring  in 
minute  but  definite  bodies  or  grains,  the  size,  shape  and  markings 
of  which  are  characteristic  and  constant  in  any  plant  species. 
The  starch  molecule  is  very  large — just  how  large  we  do  not  know 
— and  is  derived  through  the  combination  of  many  glucose  mole- 
cules, with  the  liberation  of  a  molecule  of  water  from  each,  thus : 

nCeHisOe  -  nHsO^  {CR^.O,)^. 

Neither  sugar  nor  starch  are  accumulated  in  very  great  quanti- 
ties in  the  leaf-blade,  for  most  of  the  products  of  photosynthesis 
are  removed  shortly  after  their  production  to  those  regions  of  the 
plant  where  they  are  to  be  used  or  stored. 

By-product.- — In  the  recombination  of  the  atoms  of  carbon 
dioxide  and  water  out  of  which  glucose  is  produced  there  is  evi- 
dently a  surplus  of  oxygen,  and  we  find  this  oxygen  given  off  as  a 
by-product  of  photosynthesis,  passing  forth  into  the  air  continu- 
ally from  green  plants  during  daylight.  This  is  of  little  signifi- 
cance to  the  plant  itself  but  is  often  important  to  other  organisms. 

Photosynthesis  is,  therefore,  a  constructive  process  by  which 
the  food  oj  the  plant  is  manufactured  from  very  simple  inorganic 
materials, '  through  the  agency  of  the  characteristic  substance 
chlorophyll,  and  by  energy  derived  from  fight.  The  significance 
of  photosj'nthesis  lies  in  the  fact  that  it  is  the  only  process  among 
living  things  whereby  organic  compounds  are  built  up  from  simple 
inorganic  substances,  with  the  resultant  storage  of  energ}^  All 
other  chemical  changes  in  plants  and  animals  are  concerned 
either  with  the  transformation  of  one  type  of  organic  material 

*  n  stands  for  the  unknown  number  of  smaller  molecules  which  arc  united 
into  one  of  the  large  and  complex  ones  of  such  a  substance  as  starch. 


74  BOTANY:  PRINCIPLES  AND  PROBLEMS 

into  another  or  with  the  breaking  down  of  complex  organic 
compounds  into  simpler  ones.  Photosynthesis  alone  is  funda- 
mentally constructive,  and  the  activity  of  green  plants  thus 
underlies  that  of  all  other  organisms. 

Transpiration. — The  lower  portion  of  the  mesophyll,  or  the 
spongy  layer,  is  not  concerned  primarily  with  photosynthesis 
but  with  the  interchange  of  gases  between  plant  and  atmosphere. 
Notable  among  these  interchanges  is  the  evaporation  of  water 
from  the  tissues  and  its  passage  into  the  air,  a  process  which  we 
know  as  transpiration. 

The  Importance  of  Water. — The  water-relations  of  a  plant  are 
of  the  utmost  importance  to  it  and  profoundly  influence  its 
structure  and  activities.  We  have  seen  that  water  constitutes 
the  major  portion  (75  to  90  per  cent)  of  plant  tissues  in  general, 
and  a  very  much  larger  share  of  protoplasm  itself.  An  abun- 
dance of  water  keeps  the  cells  plump,  and  by  maintaining  the 
turgidity  of  the  tissues,  enables  the  soft  parts  of  the  plant  to 
preserve  their  firmness  and  to  function  successfully.  Water  is 
one  of  the  raw  materials  entering  into  the  process  of  photosynthe- 
sis. It  is  the  solvent  of  the  mineral  nutrients,  which  can  enter 
the  plant  and  move  about  within  it  only  when  in  solution,  and 
in  watery  solutions  all  the  important  physiological  processes  of 
the  plant  take  place.  The  maintenance  of  an  abundant  supply 
of  water  in  its  tissues  is  therefore  essential  for  the  life  and  growth 
of  the  plant. 

To  this  end  the  primary  requisites  are  evidently  the  presence 
of  a  sufficient  amount  of  available  water  in  the  soil  and  its 
abundant  absorption  therefrom  by  the  roots.  Of  no  less  signifi- 
cance in  the  water-relations  of  the  plant  is  the  process  by  which 
this  water  evaporates  from  the  plant  tissues  and  passes  into  the 
air.  Absorption  must  equal  or  exceed  transpiration  if  the  plant  is 
to  thrive,  for  should  there  be  a  deficiency  in  income  or  an  excess 
of  outgo,  a  shortage  of  water  will  result  in  the  tissvies,  and  the 
plant  will  suffer  accordingly. 

Only  a  very  small  fraction  of  the  water  which  enters  the  root- 
hairs  and  passes  upward  to  the  leaves  takes  part  in  the  manu- 
facture of  sugar.  The  remainder  becomes  distributed  through 
the  cells  of  the  spongy  layer  and  evaporates  from  their  moistened 
walls,  departing  through  the  stomata  as  water  vapor  (Fig.  38). 
A  smaller  amount  may  be  evaporated  directly  from  the  surface 
of  the  epidermal  cells.     During  the  growing  season  a  constant 


THE  LEAF  AND  ITS  FUNCTIONS  75 

stream  of  water  is  thus  passing  through  the  plant  body,  entering 
at  the  root-hairs  and  leaving  through  the  stomata.  The  total 
quantity  of  this  water  often  amounts  to  several  hundred  times  as 
much  as  the  final  dry  weight  of  the  plant  itself  (Fig.  39). 


Fig.  38. — Tran.spiration  from  the  leaf-blade.  Cress-section  of  a  blade 
including  a  vein.  Arrows  indicate  passage  of  water  from  the  vein,  through 
the  mesophyll  cells,  into  the  air-spaces,  and  out  through  the  stomata.  Solid 
arrows,  liquid  water;  dotted  arrows,  water-vapor. 

The  Rate  of  Transpiration. — The  rate  of  water-loss  varies 
greatly  according  to  the  kind  of  plant,  the  soil  conditions,  the 
season  of  the  year,  the  time  of  day,  and  various  environmental 
factors.  As  a  general  rule,  we  find  that  the  rate  tends  to  increase 
under  conditions  which  favor  increased  evaporation,  such  as  high 
temperature,  bright  light,  rapid  air  movement  and  low  humidit}"; 
and  to  decrease  under  environments  of  the  opposite  character. 

Transpiration  is  by  no  means  controlled  entirely  by  factors 
which  influence  evaporation  alone.  The  rate  of  water-loss  from 
a  given  leaf-surface  and  from  an  equal  area  of  free  water  do  not 
rise  and  fall  exactly  together,  the  transpiration  from  the  living 
leaf  sometimes  being  relatively  higher  and  sometimes  relatively 
lower.  There  must,  therefore,  be  factors  in  the  leaf  itself  (as 
opposed  to  those  in  the  external  environment)  which  tend  to 
accelerate  or  to  retard  transpiration.  The  most  important  of 
these  is  doubtless  the  opening  and  closing  of  the  stomata,  which 
we  have  already  discussed.  Changes  in  the  concentration  of  the 
sap  in  the  mesophyll  cells  also  probably  determine  to  some  extent 
the  rate  at  which  water  evaporates  from  their  surfaces. 

The  actual  amount  of  water  transpired  during  the  growing 
season  may  be  large  or  small,  depending  on  the  size  of  the  plant, 


76 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


its  leaf-area,  its  transpiration-rate,  and  the  moisture  and  fertility 
of  the  soil.  Of  most  significance  to  the  plant,  however,  is  not  the 
actual  bulk  of  transpiration,  but  the  efficiency  with  which  the 
water  is  used.     This  is  determined  by  comparing  the  weight  of 


Fig.  39. — The  water-requirement  of  a  corn  plant.  The  amount  of  water 
transpired  from  this  corn  plant,  during  its  growth  and  development,  would  under 
normal  conditions  fill  the  barrel. 

the  total  water  transpired  with  the  weight  of  the  dry  plant 
material  ultimately  produced,  their  quotient  being  known 
as  the  water-requirement  of  the  plant.  Thus  when  we  say  that 
the  water-requirement  of  corn  under  certain  conditions  is  400,  we 
mean  that  for  every  gram  of  dry  weight  of  corn  plant  produced 


THE  LEAF  AND  ITS  FUNCTIONS  77 

at  maturity,  thoro  have  boon  transpired  through  its  leaves  400 
grams  of  water.  Species  vary  markedly  in  their  water-require- 
ment, and  so  do  plants  of  the  same  species  when  grown  under 
different  conditions. 

The  Significance  of  Transpiration. — Excessive  water-loss  is 
an  ever-present  danger  to  land  plants,  and  many  structural  modi- 
fications have  been  developed  by  various  species,  or  may  appear 
in  particular  individuals  growing  under  dry  conditions,  which 
tend  to  reduce  this  loss.  The  question  therefore  arises  as  to 
whether  transpiration  is  an  unmixed  evil,  made  necessary  by  the 
fact  that  the  stomata  must  be  open  to  allow  the  exchange  of 
gases  which  take  place  in  photosynthesis;  or  whether  it  is  really  a 
function  of  the  leaf  and  performs  a  useful  part  in  the  plant's 
economy.  It  was  long  thought  that  water  must  be  taken  in 
through  the  roots  in  large  quantities  to  insure  an  abundant 
absorption  of  nutrient  materials  from  the  soil,  but  a  fuller  under- 
standing of  the  phenomena  of  osmosis  and  root  absorption  shows 
the  fallacy  of  this  conclusion.  It  has  also  been  contended  that 
transpiration  is  useful  in  concentrating  the  very  dilute  solutions 
of  nutrient  salts  taken  from  the  soil — "boihng  them  down," 
so  to  speak.  We  have  seen,  however,  that  the  factors  which 
determine  the  entrance  of  nutrient  materials  into  the  plant 
preclude  such  an  explanation;  and,  indeed,  experiment  shows 
that  the  amount  of  salts  absorbed  is  practically  independent  of  the 
amount  of  water  transpired.  Transpiration  from  the  leaves, 
however,  is  evidently  what  causes  the  transpiration  stream, 
or  continuous  movement  of  water  from  root  to  leaf  through  the 
lifeless  ducts  in  the  wood  of  the  stem.  We  shall  consider  this 
movement  more  fully  when  discussing  the  functions  of  the  stem ; 
but  it  is  well  to  note  here  that  by  this  stream  the  dissolved 
substances  are  transported  bodily  from  the  central  cylinder  of 
the  root  upward  throughout  the  plant  as  far  as  the  ramifications 
of  the  dead  conducting  elements  of  the  wood  extend.  This 
movement  is  probably  far  more  rapid  than  would  result  through 
diffusion  from  cell  to  cell,  and  in  tall  plants,  particularly,  the 
transpiration  stream  probably  performs  a  distinct  service  in 
distributing  rapidly  throughout  the  plant  the  supply  of  nutrient 
materials  absorbed  from  the  soil. 

Transpiration  is  also  of  distinct  usefulness  in  regulating  the 
temperature  of  the  leaf.  The  blade  absorbs  much  more  energy 
than  it  uses  in  photosynthesis,  particularly  in  bright  light;  and 


78  BOTANY:  PRINCIPLES  AND  PROBLEMS 

the  excess,  as  heat,  would  sometimes  raise  the  temperature  of  the 
tissues  dangerously,  were  it  not  absorbed  in  evaporating  water 
from  the  mesophyll  cells. 

Transpiration  is  carried  on  primarily  in  the  leaves,  but  may 
occur  in  any  other  organs  which  are  exposed  to  the  air.  Exces- 
sive loss  of  water  is  often  prevented  in  such  regions  by  the 
development  of  cell  layers  with  corky  walls. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

200.  Are  leaves  generally  more  variable  in  their  area  or  in  their 
thickness?     Explain. 

201.  What  general  difference  in  shape  is  there  between  netted-veined 
and  parallel- veined  leaves? 

202.  Leaves  on  the  same  plant  often  differ  markedly  in  the  length 
of  their  petioles.     How  do  you  explain  these  differences? 

203.  Name  three  functions  which  the  veins  of  a  leaf  perform. 

204.  In  leaves  where  the  veins  are  much  stouter  than  the  thickness 
of  the  blade,  they  usually  stand  out  on  the  under  side  of  the  leaf  and 
thus  leave  the  upper  surface  smooth.  What  are  the  advantages  of  this 
arrangement  to  the  plant? 

205.  Of  what  advantage,  and  of  what  disadvantage,  is  it  to  plants 
of  temperate  climates  to  shed  their  leaves  in  the  winter? 

206.  The  leaves  of  "evergreen"  trees  do  not  remain  permanently  on 
the  tree  but  each  year's  crop  lives  only  a  few  seasons  and  then  drops  off. 
Why  should  these  leaves  not  continue  to  live  and  function  indefinitely? 

207.  The  upper  epidermis  and  its  cuticle  are  almost  always  thicker 
than  the  lower.     Explain. 

208.  The  cells  of  the  leaf-epidermis  usually  have  transparent  walls 
and  a  colorless  cell-sap.     Of  what  advantage  is  this  to  the  plant? 

209.  Some  parasitic  fungi  attack  only  the  epidermis  of  the  leaf,  but 
they  often  cause  the  death  of  the  leaf  and  even  of  the  entire  plant. 
Explain. 

210.  How  do  you  explain  the  fact  that  the  palisade  layer  is  next  the 
upper  surface  of  the  leaf  and  the  spongy  layer  next  the  lower? 

211.  What  does  the  plant  gain  by  having  the  cells  of  the  palisade 
layer  elongated  at  right  angles  to  the  leaf  surface? 


THE  LEAF  AND  ITS  FUNCTIONS  .  79 

212.  The  upper  surface  of  each  epideriiial  cell  of  the  leaf  Is  often 
slightly  convex.     Hoav  may  this  perhaps  be  of  advantage  to  the  plant? 

213.  Do  you  think  that  photosynthesis  is  carried  on  in  the  spongy 
layer  at  all?     What  evidence  have  you  for  your  belief? 

214.  Name  at  least  two  advantages  which  the  plant  derives  from 
having  its  stomata  more  abundant  on  the  lower  surface  of  the  leaf  than 
on  the  upper. 

215.  Stomata  are  usually  absent  in  the  leaves  of  submersed  water- 
plants.     Explain. 

216.  Why  does  washing  the  leaves  of  house  plants  often  improve  the 
health  of  the  plants? 

217.  In  cities  where  soft  coal  is  burned  and  there  is  much  coal-dust  in 
the  air,  all  trees  suffer,  and  evergreen  trees  in  particular  find  it  very 
difficult  to  hve.     Explain  these  facts. 

218.  In  what  ways  is  the  supply  of  carbon  dioxide  in  the  atmosphere 
being  replenished  ? 

219.  Do  you  think  that  the  continual  removal  of  carbon  dioxide  from 
the  atmosphere  by  green  plants  in  photosynthesis  will  reduce  the 
amount  of  this  gas  which  is  present  there?     Explain. 

220.  The  great  deposits  of  coal  formed  in  the  Coal  Period,  millions 
of  years  ago,  have  sometimes  been  pointed  to  as  evidence  that  carbon 
dioxide  was  much  more  abundant  in  the  atmosphere  then  than  now. 
What  basis  is  there  for  such  a  conclusion? 

221.  As  the  carbon  dioxide  in  the  leaf  is  removed  by  photosynthesis, 
what  causes  a  fresh  supply  to  enter  the  stomata  from  the  outside  air? 

222.  What  factors  determine  the  rate  at  which  carbon  dioxide  enters 
the  leaf? 

223.  In  some  greenhouses  the  plants  are  "fertilized"  by  pouring 
carbon  dioxide  from  a  chimney  into  the  air  around  the  plants.  Why 
does  this  increase  plant  growth  and  why  would  it  be  impracticable 
out-of-doors? 

224.  The  fact  that  carbon  dioxide  is  more  soluble  in  cold  water  than 
in  warm  has  been  cited  as  one  of  the  reasons  why  seals,  walruses,  polar 
bears,  and  even  Eskimos  are  able  to  thrive  in  such  large  numbers  so 
far  north.     Explain. 

225.  A  solution  of  chlorophyll  outside  the  plant,  when  exposed  to 
sunlight  and  in  the  presence  of  carbon  dioxide,  will  not  produce  sugar. 
How  do  you  explain  this? 


80  BOTANY:  PRINCIPLES  AND  PROBLEMS 

226.  Which  is  darker  green  in  color,  the  upper  or  the  lower  surface 
of  the  leaf?     Why? 

227.  Why  is  the  intensity  of  green  color  in  its  foliage  good  evidence 
as  to  the  health  of  a  plant? 

228.  The  stems  and  leaves  of  parasitic  plants,  such  as  mistletoe  and 
dodder,  are  either  very  pale  green  or  are  some  other  color  than  green. 
Explain. 

229.  Why  do  autumn  leaves  and  dying  leaves  often  turn  yellow? 

230.  When  cows  are  turned  into  fresh  pasture  in  the  spring,  their 
butter  turns  deep  yellow.     Why?     Why  does  it  not  turn  green? 

231.  In  order  to  "bleach"  celery  plants,  gardeners  cover  them 
with  earth  or  wrap  them  in  paper.  Why  is  this  practice  effective  in 
securing  the  desired  result? 

232.  What  is  the  orientation  (vertical,  horizontal,  oblique  or  other) 
of  an  ordinary  leaf  blade?     Explain. 

233.  The  lower  leaves  on  a  plant  or  branch  often  hang  downward 
somewhat  obliquely,  in  contrast  to  the  upper  leaves,  which  are  generally 
horizontal.     Of  what  advantage  is  this  fact  to  the  plant? 

234.  In  many  herbaceous  plants  the  lower  leaves  have  long  petioles 
but  higher  up  on  the  stem  the  petioles  gradually  decrease  in  length  and 
often  are  quite  absent  in  the  upper  leaves.     Explain. 

235.  Of  what  advantage  and  of  what  disadvantage  is  it  to  a  plant 
to  have  its  leaves  very  deeply  cut  or  lobed? 

236.  Are  opposite  leaves  usually  broader  or  narrower  than  spirally 
arranged  ones?     Explain. 

237.  Why  are  plants  with  upright,  grass-like  leaves  usually  more  suc- 
cessful in  open,  sunny  situations  than  are  plants  with  broad,  horizontal 
leaves? 

238.  Why  is  it  easier  to  maintain  a  lawn  under  elms  or  apple  trees  than 
under  maples? 

239.  In  just  what  part  of  a  tree  are  most  of  its  leaves  borne?     Why? 

240.  Why  are  the  lower  limbs  of  a  tree  growing  in  the  forest  usually 
dead? 

241.  Many  plants  are  able  to  thrive  in  relatively  deep  shade  on  the 
floor  of  the  forest,  where  the  lower  limbs  of  the  forest  trees  have  long 
since  died.     How  can  this  be? 


THE  LEAF  AND  ITS  FUNCTIONS  81 

242.  Seedlings  of  most  forest  trees  will  grow  and  thrive  on  the  forest 
floor  in  rather  deep  shade,  but  the  lower  limbs  of  mature  trees  of  the 
same  species,  if  subjected  to  similar  light  conditions,  will  soon  die.  Of 
what  advantage  to  the  plant  is  this  difference  between  young  and  old 
individuals? 

243.  Most  of  our  woodland  "wild  flowers"  (herbaceous  plants  which 
grow  on  the  forest  floor),  thrive  and  blossom  in  early  spring.     Why? 

244.  Where  does  the  light  come  from  which  is  utilized  by  the  chloro- 
phyll of  the  spongy  layer? 

245.  Leaves  exposed  to  bright  sunlight  are  thicker  than  those  grow- 
ing in  the  shade,  even  on  the  same  plant.     Explain. 

246.  How  would  you  determine  what  wave-lengths  of  light  are  of 
most  importance  in  photosynthesis? 

247.  Would  you  predict  that  a  plant  would  thrive  better  in  red  light 
or  in  green  light?     Why? 

248.  Plants  grown  in  winter  in  greenhouses  rarely  grow  as  rapidly 
as  do  plants  out-of-doors  in  the  summer,  even  though  the  temperature 
is  kept  as  high.     Explain. 

249.  Other  things  being  equal,  why  do  most  plants  grow  faster  in 
June  than  later  in  the  summer? 

250.  Crops  are  necessarily  planted  much  later  near  the  northern  limit 
of  their  range  than  farther  south,  but  in  many  cases  their  growth-rate 
is  so  much  more  rapid  that  they  reach  maturity  almost  as  soon.     Explain. 

251.  In  the  North  Sea  fisheries,  it  has  been  found  that  the  size  of  the 
season's  catch  of  fish  tends  to  be  greater  in  seasons  when  there  has  been 
much  sunshine  than  in  seasons  which  have  been  relatively  cloudy. 
Explain. 

252.  Some  plants  (such  as  the  fungi)  are  able  to  thrive  in  the  absence 
of  light.  What  other  important  phj'siological  differences  must  there 
be  between  these  and  ordinary  green  plants? 

253.  Of  what  advantage  is  it  to  the  plant  to  have  the  sugar  which  is 
produced  by  photosynthesis  converted  rapidly  into  starch? 

254.  It  is  sometimes  said  that  forests  tend  to  "])urify  the  air." 
What  basis  in  fact  is  there  for  this  statement? 

255.  Why  do  animals  in  an  acjuarium  thrive  l)ctter  if  green  water- 
plants  are  growing  in  the  aquarium? 

6 


82  BOTANY:  PRINCIPLES  AND  PROBLEMS 

256.  Compare  in  detail  the  leaf  of  a  green  plant  with  a  manufacturing 
establishment. 

257.  From  the  point  of  view  of  living  things,  which  do  you  think  are 
the  three  most  important  chemical  elements?     Why? 

258.  Which  do  you  think  would  be  worse  for  a  tree,  the  loss  of  half  of 
its  branches  by  an  ice-storm  or  of  all  of  its  leaves  through  an  insect 
attack?     Explain. 

Note. — By  the  term  dry  weight  is  meant  the  weight  of  all  the  material 
in  a  given  body  except  its  water.  It  is  usually  determined  in  the  labora- 
tory by  drying  the  material  in  an  oven  at  about  100°  C. 

259.  Would  the  dry  weight  of  a  leaf  be  greater  in  the  morning  or  in 
the  following  evening?     Why? 

260.  Will  seedlings  grown  in  the  dark  increase  in  actual  weight? 
in  dry  weight?     Explain. 

261.  The  leaves  of  parasitic  plants  are  often  very  small.     Explain. 

262.  How  do  submersed  water  plants  carry  on  photosynthesis? 

263.  Give  at  least  three  reasons  why  trees  often  fail  to  thrive  in  a  city. 

264.  From  your  knowledge  of  plants,  do  you  think  that  they  need  a 
rest  at  night,  or  would  they  thrive  continuously  if  the  light  were 
continuous? 

265.  Some  crops  produce  a  much  larger  amount  of  dry  weight  per 
acre  than  others.     Explain  how  this  can  be. 

266.  The  potato  beetle  does  not  attack  the  potato  tuber,  but  eats 
only  the  foliage  of  the  potato  plant.     Why  does  it  harm  the  crop? 

267.  Why  does  repeatedly  cutting  off  their  tops  finally  kill  persistent 
perennial  weeds? 

268.  If  a  man  wants  to  clean  out  suckers  and  low  shrubs  from  pasture- 
land,  should  he  mow  them  down  in  summer  or  in  winter?     Why? 

269.  Is  there  any  basis  in  fact  for  the  belief  held  by  some  farmers 
that  there  is  one  particular  day  in  summer  when  suckers  and  brush 
should  be  cut  down,  if  you  want  to  kill  them  off? 

270.  Which  will  give  better  and  larger  flowers  the  next  season,  tulip 
bulbs  which  are  pulled  up  immediately  after  flowering  or  those  left  in 
the  ground  until  after  the  leaves  have  withered?     Why? 

271.  Why  should  a  new  field  of  asparagus  not  be  harvested  until  two 
or  three  years  after  the  young  plants  have  been  set  out? 


THE  LEAF  AND  ITS  FUNCTIONS  83 

272.  Which  should  j^ou  keep  mown  more  closely,  a  newly  seeded  lawn 
or  an  old  one?     Why? 

273.  Should  lawn  grass,  when  cut,  be  raked  off  the  lawn  or  not? 
Explain. 

274.  Other  things  being  equal,  is  it  better  to  plant  the  rows  of  a 
garden  east  and  west,  or  north  and  south?     Why? 

275.  State  two  reasons  why  a  garden  planted  near  a  shade  tree  is  apt 
to  be  unsatisfactory. 

276.  Do  you  think  that  transpiration  is  essential  for  the  life  of  the 
plant?     Explain. 

277.  What  process  in  animals  may  be  said  to  correspond  roughly 
to  the  transpiration  of  plants?  In  what  respects  are  the  two  processes 
similar? 

278.  From  j'our  knowledge  of  osmosis,  explain  wh}'  it  is  that  "the 
amount  of  salts  absorbed  is  practically  independent  of  the  amount  of 
water  transpired  by  the  plant". 

279.  Just  where  in  the  leaf  does  evaporation  take  place  during  the 
process  of  transpiration? 

280.  What  makes  water  leave  the  cytoplasm  of  the  spongy  layer  cells 
(or  any  others)  and  wet  the  cell  walls? 

281.  Why  is  it  that  the  air  in  the  air  spaces  of  the  spongy  layer  of  the 
leaf  does  not  become  so  saturated  with  moisture  that  evaporation,  and 
consequently  transpiration,  will  no  longer  take  place? 

282.  In  general,  the  faster  a  plant  loses  water  by  transpiration,  the 
faster  it  will  absorb  it  from  the  soil.     Explain. 

283.  Do  you  think  that  a  plant  would  be  able  to  thrive  and  grow 
permanently  in  an  atmosphere  which  is  completely  saturated  with 
moisture?     Explain. 

284.  Will  transpiration  be  more  rapid  or  less  rapid  if  the  sap  of  the 
mcsophyll  cells  increases  in  concentration? 

285.  Why  does  transpiration  take  place  so  much  faster  in  wind  than 
in  still  air? 

286.  Wliy  do  florists  usually  water  the  walls  and  walks  of  a  green- 
house as  well  as  the  plants  themselves? 

287.  Why  does  a  plant  wilt  if  its  water  supply  fails? 


84  BOTANY:  PRINCIPLES  AND  PROBLEMS 

288.  Some  plants  wilt  much  more  readily  than  others.  What  factors 
may  be  responsible  for  these  differences? 

289.  Why  is  a  wilted  plant  unable  to  carry  on  its  functions  as  well  as 
one  that  is  in  a  normal,  turgid  condition? 

290.  Why  will  a  plant  sometimes  wilt  even  though  the  soil  in  which  it 
is  growing  is  abundantly  supplied  with  water? 

291.  Why  does  a  plant  which  has  wilted  in  the  daytime  usually  revive 
at  night,  even  though  no  rain  falls? 

292.  If  a  tree  is  subjected  to  a  severe  drought,  its  leaves  wilt  but  its 
twigs  remain  rigid.     Explain. 

293.  In  using  foliage  for  wreaths  or  other  decorations,  would  you 
choose  young  leaves  or  mature  leaves?     Why? 

294.  The  stomata  often  tend  to  close  in  the  middle  of  a  hot  day. 
Explain  why  this  is  so  and  how  it  is  of  advantage  to  the  plant. 

295.  Do  stomata  tend  to  open  or  close  if  the  leaf  is  exposed  to  the 
light?     Explain. 

296.  How  will  stomata  tend  to  act  during  the  course  of  a  normal 
period  of  twentj'^-four  hours  during  the  summer? 

297.  What  is  the  best  time  of  day  to  pick  flowers,  if  it  is  desired  to 
keep  them  fresh  for  a  long  time  out  of  water? 

298.  Why  will  cut  flowers  remain  fresh  longer  if  they  have  been  placed 
in  water  for  a  few  hours  directly  after  being  picked? 

299.  Do  you  think  that  the  water  requirement  of  a  plant  will  be  higher 
if  it  is  grown  on  rich  soil  or  if  it  is  grown  on  poor  soil?     Why? 

300.  Do  you  think  that  the  water  requirement  of  a  plant  will  be  higher 
if  it  is  grown  on  moist  soil  or  if  it  is  grown  on  dry  soil?     Why? 

301.  Do  you  think  that  the  amount  of  cultivation  which  a  plant 
receives  will  have  any  effect  on  its  water  requirement?     Explain. 

302.  Assuming  that  the  water  requirement  of  corn  under  a  given  set 
of  conditions  is  400,  and  that  a  bucket  holds  10  liters,  how  many  buckets 
of  water  have  passed  by  transpiration  through  a  corn  plant  the  dry 
weight  of  which  is  200  grams? 

303.  Given  the  same  conditions  as  in  Question  302,  assume  further 
that  three  such  corn  plants  are  planted  in  each  hill  and  that  the  hills  are 
one  meter  apart  each  way.  If  all  the  water  which  passes  through  these 
plants  by  transpiration  during  the  season  could  be  collected,  how  deep 
a  layer  would  it  make  over  the  surface  of  the  field? 


THE  LEAF  AND  ITS  FUNCTIONS  85 

304.  Wh}^  does  a  sliortage  of  water  stunt  a  plant? 

305.  Is  transpiration  ai)t  to  be  liif^h  or  low  when  growth  is  most  rapid? 
Explain. 

306.  Plants  generallj^  grow  faster  at  night  than  in  the  daytime. 
Why? 

307.  How  do  you  reconcile  the  fact  that  plants  grow  faster  at  night 
than  in  the  daytime,  with  the  fact  that  light  is  necessary  for  the  manu- 
facture of  food? 

308.  Plants  generally  grow  faster  in  wet  weather  than  in  dry.     Why? 

309.  Why  do  plants  often  suffer  from  "scalding"  after  a  brief  summer 
shower  in  the  middle  of  the  day? 

310.  Can  you  think  of  any  other  reason  than  the  presence  of  shade 
which  may  tend  to  make  a  forest  cool? 

311.  The  planting  of  rank-growing  species  like  the  sunflower  has 
sometimes  been  thought  to  reduce  the  amount  of  malaria  in  malarial 
districts.     What  basis  in  fact  is  there  for  this  belief? 

312.  Name  one  way  in  which  the  presence  of  a  forest  tends  to  increase 
the  loss  of  water  from  the  soil  and  one  way  in  which  it  tends  to  decrease 
it.  Do  you  think  that  a  soil  will  lose  water  faster  if  it  is  covered  with 
forest  or  if  it  is  not?     Explain. 

313.  Why  is  the  effect  of  a  killing  frost  in  the  fall  usually  not  evident 
on  the  following  morning  until  the  sun  has  been  up  for  some  time? 

314.  If  a  plant  has  been  "touched"  by  the  fi'ost  but  not  killed,  whj' 
is  it  sometimes  possible  to  restore  it  by  sprinkling  it  with  water  and 
placing  it  in  a  cool,  shaded  place? 

315.  What  structural  modifications  do  .you  know  of  in  plants  which 
tend  to  result  in  checking  transpiration? 

316.  What   characteristics   must   drought-resistant   plants   possess? 

317.  The  leaves  of  corn  and  other  grasses  often  roll  or  curl  when  it  is 
very  hot  or  dry.  Why  is  this  and  of  what  advantage  may  it  be  to  the 
plant? 

318.  The  leaves  of  desert  plants  are  usually  leather)',  fleshy  of  very 
small,     l^kplain. 

319.  The  leaves  of  Eucalyptus,  a  tree  which  floui'ishes  in  warm,  dry 
regions,  hang  vertically  on  the  branches.  Explain  how  this  is  of 
advantage  to  the  plant. 


86  BOTANY:  PRINCIPLES  AND  PROBLEMS 

320.  The  leaves  of  "compass  plants"  are  erect  and  vertical,  with 
their  edges  pointing  approximately  north  and  south.  In  what  two  ways 
may  this  character  be  of  advantage  to  the  plant? 

321.  Plants  with  downy  or  woolly  leaves  are  very  abundant  on 
mountain  sides  and  exposed  alpine  situations.  Why  are  they  particu- 
larly suited  to  such  conditions? 

322.  Why  do  epiphytes  (p.  174)  usually  have  leathery  leaves? 

323.  The  leaves  of  evergreens  in  temperate  regions  are  usually  firm 
and  leathery.     Explain. 

324.  Evergreen  trees  often  suffer  from  "wind  burn"  in  late  winter  or 
early  spring,  a  part  or  all  of  their  branches  dying  and  turning  brown. 
They  do  not  suffer  from  this  cause  earlier  or  later  in  the  season. 
Explain. 

325.  Give  two  reasons  for  the  fact  that  plants  grown  in  the  shade 
are  usually  more  tender  than  plants  grown  in  the  bright  sunlight. 

326.  Why  is  the  best  lettuce  grown  in  early  spring? 

327.  Spring  is  the  best  season  to  transplant  a  tree.     Why? 

328.  Why  is  it  better  to  do  transplanting  on  a  cloudy  or  rainy  day 
than  on  a  bright  one? 

329.  Why  is  it  advantageous  to  cut  off  the  outer  leaves  of  young 
plants  before  transplanting? 

330.  If  it  is  necessary  to  transplant  young  trees  during  the  summer, 
the  plants  should  first  be  vigorously  pruned.     Why? 

331.  Why  will  potatoes  in  storage  lose  weight  much  faster  after 
sprouts  have  grown  out  on  them? 

332.  Why  is  it  best  to  store  apples  in  a  fairly  moist  place  but  beans 
where  it  is  dry? 

333.  Tobacco  growers  sometimes  cover  their  plants  with  cheese-cloth 
tents.  What  effect  do  you  think  this  has  on  the  structure  and  functions 
of  the  tobacco  leaves? 

REFERENCE  PROBLEMS 

36.  Give  an  example  of  a  leaf  which  has  assumed  some  of  the  functions 
of  a  stem. 

37.  Where  is  the  palisade  layer  in  leaves  which  stand  erect,  like  those  of 
Iris? 


THE  LEAF  AND  ITS  FUNCTIONS  87 

38.  Wliat  various  functions  may  stipules  i:»crform? 

39.  Give  the  number  of  stomata  found  per  square  centimeter  of  leaf 
surface  on  three  species  of  plants. 

40.  What  is  the  composition  of  ordinary  air? 

41.  How  many  cubic  centimeters  of  air  are  needed  to  provide  enough 
carbon  dioxide  for  the  manufacture  of  one  gram  of  starch  by  photosynthesis? 

42.  About  what  proportion  of  the  energy  reaching  a  leaf  in  sunlight  is  used 
in  the  process  of  photosynthesis?  How  does  this  compare  with  the  amount 
utilized  by  a  good  steam-engine  from  the  burning  of  coal? 

43.  About  how  much  sugar  is  normally  manufactured  by  one  square  meter 
of  leaf-surface  of  an  average  plant  during  an  average  summer's  day  ? 

44.  Are  the  leaves  of  "foliage-plants,"  which  are  brightly  colored  (not 
green)  able  to  carry  on  photosynthesis?     Explain. 

45.  In  the  case  of  some  one  common  crop-plant,  give  the  percentage  of 
dry  matter,  of  water,  and  of  ash  which  typically  compose  it.  From  what 
source  has  each  been  derived? 

46.  When  and  by  whom  was  the  process  of  photosynthesis  in  plants  first 
clearly  understood? 

47.  What  is  the  average  annual  rainfall  in  this  region?  About  how  much 
of  this  is  returned  to  the  air  by  a  vigorous  crop  through  transpiration? 

48.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Petiole  Stoma  Xanthophyll 

Epidermis  Photosynthesis  Chloroplast 

Mesophyll  Chlorophyll  Transpiration 


CHAPTER  VI 
THE  STEM  AND  ITS  FUNCTIONS 

We  have  shown  that  the  root,  which  absorbs  water  and 
mineral  substances  from  the  soil,  and  the  leaf,  which  carries 
on  the  manufacture  of  food,  are  the  primary  vegetative  organs 
of  the  plant.  A  third  member,  the  stem,  connects  these  two. 
It  forms  a  conspicuous  feature  of  most  plants  and  in  woody 
species  constitutes  the  great  bulk  of  their  bodies.  Its  functions, 
though  secondary  to  the  major  activities  which  we  have 
mentioned,  are  nevertheless  essential  ones.  It  serves  to  dispose 
the  leaves  in  situations  favorable  for  photosynthesis,  and  provides 
a  highway  for  transportation  between  leaf  and  root.  In 
addition,  the  stem  frequently  becomes  a  storage-organ  and  may 
be  variously  modified  for  other  special  functions. 

The  External  Structure  of  the  Stem. — The  stem  displays  a 
wide  range  of  variation  in  size  and  in  external  and  internal  struc- 
ture, according  to  the  habit  or  growth-form  which  the  plant 
assumes.  In  herbs  (Fig.  40),  where  the  whole  shoot  dies  back 
to  the  ground  during  periods  unfavorable  to  vegetative  activity 
or  at  the  completion  of  a  given  cycle,  the  stem  is  comparatively 
slender  and  soft  in  texture.  In  plants  with  perennial  above- 
ground  parts,  however,  it  grows  thicker  from  year  to  year  and 
becomes  hard  and  woody,  forming  the  stout  stems  characteristic 
of  shrubs  (Fig.  41)  and  trees  (Fig.  42).  In  shrubs  the  stem  is 
comparatively  short  and  slender  and  is  usually  much  branched, 
even  close  to  the  ground.  In  trees,  it  grows  taller  and  is  devel- 
oped for  some  distance  upward  into  a  main  stem  or  trunk  which 
may  become  very  thick.  Woody  stems  transitional  between  these 
two  types  often  occur. 

Buds. — The  growth  of  the  stem  in  length  takes  place  only  at 
certain  definite  points,  where  the  cells  are  thin-walled  and  capable 
of  active  division.  In  many  stems,  particularly  those  which  are 
perennial  and  woody,  these  growing-points  are  protected  by  leaves 
or  scales  and  are  then  known  as  buds  (Fig.  43).  Buds  may  be 
terminal,  developing  at  the  tip  of  the  stem,  or  lateral,  arising  from 


THE  STEM  AND  ITS  FUNCTIONS 


89 


the  sides.  Within  the  biul  are  not  only  the  beginnings  of  the 
young  stem  but  of  the  various  structures  which  arc  borne  upon  it, 
such  as  leaves  and  flowers.  The  bud  scales,  which  protect  these 
delicate  parts,  are  usually  stout  and  impervious. 


pur  {Dcljihiniinn). 


The  terminal  bud  governs  the  elongation  of  the  stem,  and 
through  the  development  of  lateral  buds,  branches  arise.  The 
shape  of  the  aerial  portion  of  the  plant  is  determined  primarily 
by  the  number  and  arrangement  of  these  branches  and  by  th(Mr 
rate  of  growth  relative  to  each  other  and  to  the  main  st  (mu. 


90 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


In  certain  herbaceous  plants  the  terminal  bud  produces  a 
flower  or  flower-cluster,  and  the  growth  of  the  stem  in  length 
usually  ceases  at  this  point.  Such  deterininaie  growth  is  not 
common  among  wood}^  plants,  however,  and  their  stems  continue 
to  elongate  indefinitely. 

Leaves.- — Leaves  are  borne  throughout  the  length  of  the  stem 
in    herbaceous  plants  and  on  the  twigs  of  the  current  year's 


m^^ljUj^^ 

l^^i 

^ 

mi 

^^m 

"  ^^PSp^BBPWPw^^HliPIIPVi^Hi 

I  I.,.    41.      A.^hiiih.      'I'hi^  Uhiv  {SyruKja  vubjarci). 

growth  in  woody  forms  (Fig.  44).  That  point  on  a  stem  at  which 
a  leaf  is  attached  is  called  a  node  and  the  region  between  two  nodes, 
an  internode  (Fig.  43).  The  position  of  the  node  also  governs  the 
position  of  the  lateral  bud,  for  such  a  bud  normally  arises  only 
in  the  leaf  axil,  or  upper  angle  between  leaf  and  stem. 

The  arrangement  of  leaves  on  the  stem,  or  its  phyllotaxy, 
may  display  many  different  types.  If  but  one  leaf  occurs  at  a 
node  the  next  one  above  it  arises  from  the  other  side  of  the  stem, 
and  the  arrangement  is  thus  an  alternate  one  (Fig.  44).     These 


THE  STEM  AND  ITS  FUNCTIONS 


91 


two  leaves  may  be  exactly  half  way  around  the  stem  from  each 
other,  but  it  is  much  more  common  for  their  angle  of  divergence 
to  be  less  than  180°  and  for  the  points  of  attachment  of  a  series  of 
successive  leaves  thus  to  form  a  loose  spiral  around  the  stem. 
The  closeness  of  this  spiral  and  the  position  of  the  leaves  thereon 
show  great  diversity,  but  are  generally  constant  within  any  par- 


FiG.  42.— A    tree. 


The    shagbark    hickory    (Carya    ovata). 
States  Forest  Service) . 


{Courtesy    United 


ticular  species.  If  two  leaves  arise  from^the  same  node  they  are 
always  directly  across  the  stem  from  each  other  and  are  said  to 
be  opposite  in  arrangement  (Fig.  43).  When  there  are  more 
than  two  leaves  at  a  node,  they  are  disposed  about  the  stem  in  a 
circle  or  whorl. 

Surface. — The  surface  of  a  young  stem  is  protected  only  by  an 
epidermis,  but  later  this  is  replaced  in  wood}-  plants  bj^  a 
characteristic  layer  of  corky  cells,  the  bark.  The  necessary 
exchange  of  gases  between  the  air  and  the  living  tissues  of  the 


92 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


stem  takes  place  through  the  lenticels  (Fig.  43),  small  spots  or 
strips  where  the  bark  tissue  is  softer  and  looser  than  elsewhere. 


Terminal 
Bud 


Lateral 
Bud 


I: 


il   ' 


-Internode 


Leaf 
Scar 


Node 


«^-#L^'f  Scar  of 

Terminal 
Bud 


Lenticel 


Bundle 
Scar6 


Fig.  43. — A  woody  twig  in  winter  condition  {Horse-chestnut) . 


Other  Stem  Types. — The  typical  upright,  foliage-bearing  stem 
has  sometimes  become  radically  modified  for  the  performance  of 


THE  STEM  AND  1T,S  FUNCTIONS 


93 


other  functions  than  support  and  {'oiuhiction.  Many  plants 
have  abandoned  the  erect  habit,  and  their  weak,  slender  stems 
climb  or  scramble  by  various  means  over  other  objects  or  lie 
prostrate  on  the  ground.  In  certain  herbs  the  main  stem  may 
even  become  subterranean,  in  which  condition  it  is  known  as  a 


A  B 

Fig.  44. — Summer  (B)  and  winter  (^4)  eoiiditionof  the  same  woody  twig  (Cherry). 
The  buds  arise  in  the  angle  between  leaf  and  steam. 


rooisiock  or  rhizome  (Fig.  45).  Typical  stems  give  opportunity 
for  the  storage  of  a  certain  amount  of  food  reserves,  especially 
in  pith  and  cortex,  but  in  some  species  this  function  is  so  greatly 
developed  that  the  stem  system,  or  certain  parts  of  it,  becomes 
essentially  a  storage  organ  only.  This  condition  exists  in  most 
rootstocks,  and  its  extreme  development  results  in  a  reduction 
of  the  stem  to  a  short,  thick  tuber  such  as  we  know  in  the  potato 
(Fig.  46),  which  is  morphologically  a  stem  but  now  shows  little 
obvious  resemblance  to  that  organ.  The  bulb  and  the  corm 
(Fig.  47)  are  other  examples  of  highly  modified  underground  stems. 


94 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


The  Internal  Structure  of  the  Stem. — In  the  cross  section  of  a 
typical  young  stem  (Figs.  48,  53  and  54)  there  may  be  distin- 
guished the  same  three  types  of  tissue  which  are  present  in  the 


Fig.   45. — Rootstock  of  Iris. 


Fig.  46. — Tuber  of  the  potato,  fehowmj;  point  of  ittithrneiit  to  the  parent 
plant  (at  extreme  light)  and  numerous  budb  or  eje&,'  eacli  in  tht  axil  of  a 
reduced  and  scale-like  leaf. 


root,  but  they  are  arranged  somewhat  differently.  Outside 
the  whole  is  the  epidermis,  consisting  of  a  single  cell-layer,  and 
often  replaced  entirely,  at  an  early  stage,  by  a  zone  of  corky  bark. 


THE  STEM  AMJ  ITS  FUNCTIONS  95 

Beneath  this  is  the  cortex,  varying  in  thickness  ])ut  rare!}'  oceupy- 
ing  as  prominent  a  place  in  the  stem  as  it  does  in  the  root. 
Beneath  the  cortex  hes  the  jibro-vascular  cylinder  which,  iinhke 
its  counterpart  in  the  root,  is  arranged  in  the  form  of  a  hollow 
tube.     The  core  of  this  tube  is  occupied  by  the  yith,  a  tissue 


Fig.  47. — Bulb  and  corm.  {A),  longitudinal  section  through  the  bulb  of  a 
hyacinth.  The  short,  broad  stem  bears  a  cluster  of  fleshy  leaves,  the  central  ones 
of  which  grow  out  as  foliage  leaves.  {B) ,  longitudinal  section  through  the  corm  of 
a  crocus,  showing  the  thick,  short  stem  surrounded  by  the  fibrous  remains  of  old 
leaf-bases.  The  remains  of  corms  of  three  preceding  seasons  are  shown  below 
the  present  one.      The  tops  of  the  leaves  have  been  cut  off  in  both  illustrations. 

much  resembling  the  cortex.  A  more  detailed  account  of  the 
character  of  the  cells  composing  these  tissues  may  be  appropri- 
ately undertaken  now,  for  although  all  the  tissues  here  men- 
tioned are  present  in  root,  stem,  and  leaf,  they  reach  their  greatest 
differentiation  and  complexity  in  the  stem,  and  in  this  region  of 
the  plant  they  can  therefore  most  profitably  be  studied.  The 
structure  of  the  fibro-vaseular  tissues  of  a  wood}-  dicotyledonous 
plant  can  well  be  seen  in  Figs.  49 "and  50,  a  transverse  and  a  radial 
longitudinal  section  through  a  portion  of  the  stem  shown  in 
Fig.  48. 


96 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Protective  Layers. — The  epidermal  cells  resemble  those  of 
the  leaf  epidermis  and  require  no  special  comment.  In  stems 
which  are  growing  in  thickness,  however,  the  epidermis  is  soon 
sloughed  off  and  its  protective  function  is  assumed  by  a  layer 
of  corky  cells  formed  directly  under  it  and  constantly  renewed. 
In  these  cells  the  protoplasm  soon  disappears  and  the  normal 


Fig.  48. — Tranfe\ei-5e  section  of  a  thicc-\  c  ii-oUl  twig  of  the  tiilip-tree  {Lirio- 
dendron).  This  is  a  t>pical  wood>  twig  The  hbro-vascular  cylinder  consists 
of  a  solid  ring  of  wood  within  and  ba^t  without,  surrounding  a  central  pith. 


cellulose  wall  becomes  corky  or  suherized  and  is  thus  rendered 
almost  impermeable  to  air  or  water.  The  lenticels,  which  we 
have  already  mentioned,  are  spots  in  this  corky  layer  where  the 
cells  are  somewhat  loose  and  spongy  and  thus  allow  the  passage 
of  gases. 

Cortex  and  Pith. — The  cortex  and  the  pith  are  very  similar 
in  constitution.  Their  cells  usually  remain  alive,  are  roughly 
spherical  in  shape,  retain  their  cellulose  walls  and  function 
chiefly  in  the  storage  of  food.  To  such  undifferentiated  tissues 
the  term  parenchyma  is  often  applied.  In  older  woody  stems  the 
pith  often  dries  up  and  collapses;  and  the  cortex,  crushed  by  the 
expansion  of  the  wood  underneath  it,  is  finally  sloughed  off. 


THE  STEM  AND  ITS  FUNCTIONS 


97 


Bast. — The  fibro-vascvilar  e3'liii(ler  is  composed  of  two  tlistinct 
tissues.  On  the  outside  is  the  bast  or  phloem,  the  function  of 
which  is  to  transport  the  elaborated  foods — the  carbohydrates, 


--Bast  Fibers 
^7-- Sieve  Tube 

Cambium 


Wood  Ray 


-  Wood  Fibe- 


Fig.  49. — Transvert^e  .seciion  of  wood  and  bast  of  the  tulip-tree  (Liriodendron) . 
A  portion  of  Fig.  48  more  highly  magnified,  including  part  of  one  of  the  segments 
between  two  wood-rays.  The  last  annual  ring  of  wood,  together  with  one  of  the 
groups  of  bast  cells,  is  included. 


fats,  and  proteins — from  one  part  of  the  plant  to  another, 
especially  from  regions  of  manufacture  to  those  of  storage  or 
consumption.  The  cells  concerned  in  this  process  are  the  sieve- 
tubes  (Fig.  51),  living  cells  with  thin  cellulose  walls  but  unique 


98 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


in  their  lack  of  a  nucleus.  They  are  elongated  parallel  to  the 
main  axis  of  the  stem  and  their  end  walls  (more  rarely  their 
sides)  are  provided  with  sieve-plates  or  definite  groups  of  small 
perforations.     Through    these    perforations    extend  threads  of 


Cambium 


IVood 


Bast 


Companion 
Cell 


.Bast 
Parenchyma 


Sieve 
Cambium         Fibers  Tubes 

Fig.  50. — Radial  longitudinal  section  of  wood  and  bast  of  the  tulip-tree 
{Liriodcndron) .  Section  cut  through  the  region  shown  in  Fig.  49.  At  the  left 
is  the  wood,  and  at  the  right,  the  bast,  with  the  canilMum  between.  The  ladder- 
like markings  in  the  vessels  are  the  end  walls  of  the  vessel-cells,  and  the  small, 
elliptical  markings  are  pits  in  the  side  walls.  The  ends  of  the  sieve-tubes  are 
occupied  l)y  sieve-plates. 


Vessels 


Bast 
Fibers 


cytoplasm  from  one  cell  to  another,  so  that  the  living  substance 
of  each  sieve-tube  is  directly  continuous  with  that  of  the  adjacent 
ones.  In  the  highest  seed  plants  there  is  next  to  each  sieve-tube 
a  small  companion  cell,  provided  with  an  abundance  of  cytoplasm 
and  a  nucleus.  In  addition  to  these  two  types,  groups  of  long 
and   very  thick-walled   cells,   the   hast-fibers,   characteristically 


THE  STEM  AND  ITS  FUNCTIONS 


99 


Fig.  51  —The  strut-ture  of  a  sieve-tube  (Squash).  .1,  longitudinal  section  of 
a  sieve-tube,  communicating  bj-  sieve-plates  with  the  adjacent  sieve-tubes  above 
and  below.  Its  companion-cell  is  at  the  left.  B,  transverse  sections  through 
two  sieve-tubes.  The  one  at  the  left  is  cut  near  the  middle.  The  one  at  the 
right  IS  cut  near  the  end,  its  end  wall,  or  sieve-plate,  showing  in  the  section 


100 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


occur  in  the  phloem,  and  some  parenchyma  is  usually  present 
there  also. 

Wood. — The  inner  portion  of  the  fibro-vascular  cylinder 
consists  of  the  wood  or  xyleyn,  which  provides  mechanical  rigidity 
for  the  stem  and  transports  the  stream  of  water  and  dissolved 
substances  from  root  to  leaf.     As  essential  elements  in  the  xylem 


Fig.  52. — Types  of  cells  found  in  wood.  A,  fiber.  B,  tracheid.  C,  vessel- 
cell,  with  ladder-like  end  wall.  D,  vessel-cell,  with  completely  open  or  porous  end 
wall.  E,  very  short,  broad,  vessel-cell.  F,  two  ray-cells.  G,  two  vertical  wood- 
parenchyma  cells. 


we  find  cells  which  are  much  elongated  parallel  to  the  main  axis 
of  the  stem  and  in  which  the  cellulose  walls  have  become  very 
thick  and  woody  (Fig.  52,  A,  B,  C,  D  and  E).  Such  walls  are 
said  to  be  lignified.  As  soon  as  one  of  these  cells  is  fully  devel- 
oped, it  dies  and  its  protoplasmic  contents  disappears,  so  that  only 


THE  STEM  AND  ITS  FUNCTIONS  101 

the  thick,  woody  cell-wall  is  left.  Definite  thin  areas  or  pits 
occur  at  frequent  intervals  along  this  wall  and  facilitate  the  rapid 
movement  of  water.  In  simpler  types  of  wood,  such  a  cell  is 
able  to  provide  both  the  necessary  rigidity  and  conductive 
capacity  and  is  known  as  a  tracheid.  In  the  higher  types,  how- 
ever, this  simple  element  has  become  specialized  in  two  directions 
and  has  given  rise  to  long  and  very  heavy-walled  cells,  the 
wood-fibers,  in  which  almost  no  cavity  remains  and  which  con- 
tribute a  high  degree  of  mechanical  strength  to  the  wood;  and 
the  vessel-cells  or  tracheal  cells,  much  shorter,  with  wide  cavities, 
and  walls  which  are  comparatively  thin  and  are  provided  with 
large  perforations  in  their  ends.  These  cells,  laid  end  to  end  in 
vertical  rows,  constitute  the  ducts  or  vessels,  so  characteristic  of 
the  wood  of  many  plants,  which  carry  the  ascending  stream  of 
water  through  the  stem.  Parenchyma  cells  sometimes  occur 
among  the  lignified  elements  and  like  them  may  be  elongated 
vertically  (Fig.  52,  G).  Other  parenchyma  cells  are  elongated 
at  right  angles  to  the  stem  (Fig.  52,  F)  and  dispersed  among  the 
woody  cells  in  horizontal  bands  or  ribbons  running  out  through 
the  xylem  along  the  radii  of  the  stem.  These  structures  are 
known  as  the  wood-rays,  and  in  somewhat  modified  form 
extend  also  into  the  bast.  They  facilitate  the  horizontal 
transfer  of  materials  in  the  stem  and  are  of  particular  importance 
as  centers  of  food-storage. 

Cambium. — A  narrow  layer  of  thin-walled  cells,  the  cambium, 
separates  the  wood  from  the  bast.  Through  its  activity  new  cells 
are  added  to  the  outside;  of  the  wood  and  the  inside  of  the  bast, 
and  the  thickness  of  the  stem  is  thereby  increased.  Among 
woody  plants,  such  growth  continues  from  year  to  year  and  each 
season's  increment,  or  annual  ring,  is  easily  recognizable. 

At  each  node  a  small  but  complete  segment  of  the  fibro- 
vascular  ring  separates  from  the  rest  and  passes  out  through  the 
cortex  into  the  base  of  the  petiole,  causing  a  break,  or  leaf-gap, 
in  the  ring.  Into  each  leaf  may  enter  one,  three,  five  or  more  of 
these  leaf-traces  which  are  destined  to  pass  upward  through  the 
petiole  and  to  form  the  system  of  veins  in  the  blade. 

Woody  and  Herbaceous  Stems. — The  perennial  woody  stem  in 
which  the  fibro-vascular  cylinder,  as  seen  in  cross  section,  forms  a 
continuous  and  rather  wide  ring  (except  for  the  leaf-gaps),  and 
which  receives  adcUtions  in  thickness  year  by  year  through 
cambial  activity,  is  probably  the  most  ancient  stem-type  among 


102  BOTANY:  PRINCIPLES  AND  PROBLEMS 

seed  plants;  and  the  herbaceous  condition,  where  the  stems  are 
much  softer  and  shorter-lived,  has  apparently  been  derived 
from  it  in  response  to  climatic  changes  or  for  other  reasons. 
In  herbaceous  species,  the  amount  of  fibro-vascular  tissue  has 
become  proportionally  very  much  less.  This  may  be  due  simply 
to  a  decrease  in  the  activity  of  the  entire  cambium,  or  to  the 
breaking  up  of  the  cylinder  into  separate  bundles,  but  in  general 


Fig.  53,  Fig.  54. 

Fig.  53. — Transverse  section  of  a  one-year-old  twig  of  the  sweet  gum  {Liquid- 
amhar),  showing  the  continuous  fibro-vascular  cylinder. 

Fig.  54. — Transverse  section  of  a  one-year-old  twig  of  the  sycamore  (Platanus), 
showing  the  fibro-vascular  cylinder  broken  up  into  segments  by  the  development 
of  wide  rays.    ■  {Figs.  53  and  54  from  Sinnott  and  Bailey). 

any  herbaceous  stem  is  roughly  comparable  to  a  one-year-old 
twig  of  the  particular  woody  stem-type  from  which  it  has  been 
evolved.  The  herbaceous  stem  in  Fig.  55,  with  its  thin  but 
continuous  vascular  ring,  has  probably  arisen  from  some  such 
woody  form  as  is  shown  in  Fig.  53,  where  the  vascular  ring  is 
similarly  continuous  and  homogeneous.  The  stem  in  Fig.  56, 
however,  in  which  the  cylinder  has  been  broken  into  distinct 
and  completely  separate  bundles,  is  quite  different  in  type  and 
has  probably  arisen  from  a  woody  stem  somewhat  resembling 
that  in  Fig.  54,  where  the  vascular  ring  is  divided  into  segments 
by  the  development  of  very  wide  rays.  Cambial  activity  is 
usually  weaker  opposite  these  rays  than  opposite  the  woody 
segments  of  the  cylinder,  and  in  the  stouter  herbs  of  this  type, 


THE  STEM  AND  ITS  FUNCTIONS 


103 


Fig.  55.  Fig.  :>ii. 

Fig.  55. — The  stem  of  an  herbaceous  plant  (Digitalis).  Transverse  section. 
In  this  type  of  herbaceous  stem,  the  fibro-vascular  cylinder  is  thin  but  unbroken. 

Fig.  56. — The  stem  of  an  herbaceous  plant  {Delphinium).  Transverse 
section.  In  this  type  of  herbaceous  stem,  the  fibro-vascular  cylinder  is  broken 
into  a  ring  of  fibro-vascular  bundles,  each  containing  a  group  of  wood-cells  and  a 
group  of  bast-cells. 


Fig.   57. — The  stem  of  a  monocotyledonous  plant  (Corn).      Transverse  section, 
showing  the  fibro-vascular  bundles  scattered  in  the  pith. 


104  BOTANY:  PRINCIPLES  AND  PROBLEMS 

the  rays  therefore  tend  to  form  broad  constrictions  in  the  ring. 
In  more  dehcate  herbaceous  stems  the  constrictions  finally 
become  complete,  the  broad  rays  disappear,  and  the  cylinder  is 
thus  broken  up  into  a  ring  of  separate  segments  or  fibro-vascular 
hmidles.  Each  of  these  consists  of  a  group  of  wood  cells  on  its 
inner  side  and  of  bast  cells  on  its  outer,  with  a  vestige  of  cambium 


Fig.  58. — Stem-bundle  of  a  monocotyledonous  plant.  Transverse  section 
of  a  fibro-vascular  bundle  from  the  Corn  stem  (Fig.  57).  A,  vessels.  B,  sieve- 
tubes,  with  companion-cells  in  their  corners.  The  bundle  is  surrounded  by  a 
bundle-sheath  of  thick-walled  cells. 

between.  Connecting  the  cambium  layers  of  two  adjacent 
bundles  there  may  be  a  weak  interfascicular  cambium,  producing 
a  few  layers  of  parenchyma  cells.  In  many  herbaceous  stems, 
however,  the  bundles,  each  surrounded  by  a  bundle-sheath  of 
thick-walled  cells,  are  quite  distinct  and  widely  separated  from 
one  another,  with  no  remnant  whatever  of  a  cambial  zone  between 
them. 

In  still  more  highly  specialized  stems,  characteristic  of  mono- 
cotyledonous plants,  the  bundles  are  no  longer  arranged  in  a  ring 
but  are  scattered  irregularly  throughout  the  whole  area  of  the 


THE  STEM  AND  ITS  FUNCTIONS  105 

stem  (Fig.  57).  The  individual  Inindlcs  are  very  distinctive 
in  appearance  (Fig.  58),  each  possessing  a  large  air-space  or 
lacuna,  two  large  vessels,  and  a  patch  of  very  regularly  arranged 
sieve  tubes  and  companion  cells.  In  such  a  stem  no  distinction 
between  pith  and  cortex  now  remains.  The  departure  of  the 
leaf-traces  here  is  very  complex,  a  large  number  of  bundles 
moving  outward  from  the  center  of  the  stem  and  entering  the 
sheathing  leaf-base. 

The  Structure  of  Wood. — In  shrubs  and  trees*  the  great  bulk  of 
the    stem,    particularly    in   its    older    portions,   consists   of    but 


^'  -""^  xSap-Wood 


i 


iiSfm0  >Heart-W/ood 


Fig.  59. — Heart-wood  and  sap-wood.  Transversely-cut  end  of  an  oak  log, 
showing  the  darkly-colored  heart-wood  at  the  center  of  the  stem,  surrounded  by 
the  lighter  sap-wood. 

one  tissue,  the  wood.  Wood  is  so  important  in  the  economy 
of  the  plant  and  of  such  great  significance  to  man  that  we  are 
justified  in  studying  it  a  little  more  closely  than  we  have  the 
other  tissues. 

Through  the  activity  of  the  cambium  (a  fuller  account  of  whicli 
we  shall  reserve  for  the  chapter  on  growth)  a  new  concentric 
layer  of  wood  cells  is  added  each  year  to  the  outside  of  the  woody 
cylinder.  The  tracheids  and  ducts  produced  at  the  beginning 
of  growth  in  the  spring  are  usually  of  large  diameter  and  are 
accompanied  by  comparatively  few  fibers,  and  it  is  apparently 

*  Conifers  and  dicotyledons  alone  are  discussed  here.  Woody  mono- 
cotyledons are  rare  and  their  woody  tissues  are  very  complex. 


106 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


in  this  spring  wood  that  most  of  the  upward  conduction  of  water 
takes  place.  In  the  later-formed  portion  of  the  annual  ring,  the 
water-carrying  cells  are  fewer  and  narrower,  and  the  bulk  of  the 
tissue  is  composed  of  fibers.  This  summer  wood  is  responsible 
for  most  of  the  rigidity  and  strength  of  the  stem.  In  large 
branches  and  trunks,  the  older  portion  of  the  wood,  consisting  of 
the  first-formed  annual  rings  at  the  center  of  the  stem,  in  time 
becomes  dead  throughout  and  ceases  to  perform  its  functions  of 


Transverss 


Radial 


Tangential 

Fig.  60. — Transverse,  tangential,  and  radial  sections  of  wood.  Both  logs  have 
been  cut  transversely.  In  addition,  the  one  at  the  left  has  been  cut  lengthwise 
tangentially,  and  the  one  at  the  right,  lengthwise  radially.  Each  cut  presents 
the  appearance  characteristic  of  that  particular  plane  of  section.  The  -annual 
rings  and  wood  rays  may  be  distinguished  in  all  three  sections. 


water-conduction  and  storage.  It  then  constitutes  the  heart- 
wood  (Fig.  59)  and  is  frequently  distinguished  from  the  outer 
layers  by  its  darker  color.  The  living  and  functioning  part  of  the 
wood  is  its  youngest  portion  and  is  known  as  the  sap-wood  (Fig. 
59).  This,  of  course,  is  on  the  outside  of  the  woody  cylinder, 
and  it  is  usually  rather  constant  in  width  in  any  particular  species, 
its  innermost  ring  being  converted  into  heart-wood  each  year  as 
its  outermost  is  added  by  the  activity  of  the  cambium.  All 
of  the  non-woody  cells  here  (the  parenchyma  cells  and  ray  cells) 
are  alive. 

Wood  is  usually  cut  along  one  of  three  distinct  planes,  and  the 
cut  surface  in  each  case  presents  a  very  different  appearance 
(Fig.  60).  In  describing  a  given  wood  it  is  therefore  customary 
to  consider  its  characteristics  as  they  are  shown  in  these  three 


THE  STEM  AND  ITS  FUNCTIONS 


107 


cuts  or  sections.  An  ordinal y  "cross  cut,"  at  right  angles  to  the 
length  of  the  log,  is  known  as  a  transverse  section,  and  shows  the 
annual  rings  as  a  series  of  concentric  circles,  with  the  wood  rays 
running  out  from  the  center  as  narrow  lines  along  the  radii. 
Whore  the  cut  is  longitudinal  and  made  exactly  along  the  radius 
of  the  log,  a  7'adial  section  results.  This  presents  the  annual 
rings  as  vertical  straight  lines  and  the  wood  rays  as  horizontal 


..Large 
Wood  Rqij 


Spring 
Vessels 


Annual 
Ring 


Fig.  61. — A  segment  of  an  oak  log.  At  the  right,  the  block  has  been  cut 
radially,  and  above,  transversely.  At  the  left  (the  surface  of  the  log)  a  portion 
of  the  bast  and  corky  bark  has  been  removed,  showing  a  tangential  view  of 
the  wood  beneath.  The  wood  rays  are  narrow  sheets  of  tissue  running  along  the 
radii  of  the  stem.  On  the  radial  face,  one  of  them  is  shown  split  open,  giving  the 
characteristic  "silver  grain"  of  oak  wood.  Between  the  large  rays  are  many 
small  and  narrow  ones. 


stripes  or  markings.  Where  the  rays  are  fairl}-  wide,  as  in  the 
oak,  these  markings  are  prominent  and  furnish  the  much-prized 
"  silver  grain  "  so  readily  seen  in  quartered  oak.  Other  longitudi- 
nal sections,  which  do  not  lie  in  a  plane  passing  through  the  center 
of  the  log,  are  known  as  tangential.  If  the  structure  is  exactly 
regular  and  the  cut  exactly  true,  the  annual  rings  are  here  seen  as 
straight  lines  somewhat  unequally  distant,  running  up  and  down 
along  the  wood.  The  irregularities'  which  almost  always  occur, 
however,  cause  the  rings  in  such  a  cut  to  appear  as  wavy  lines 
which  produce  the  common  "grain"  of  most  wood  surfaces. 
The  rays  are  very  inconspicuous  in  a  tangential  section,  for 
only  their   cut   ends  are  visible.     The  relations  between  these 


108 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


three  sections,  and  the  characteristic  appearance  of  the  various 
wood  structures  as  seen  in  them,  is  shown  in  the  segment  of  an 
oak  log  (Fig.  61)  and  the  magnified  cube  of  pine  wood  (Fig.  62). 
Woods  of  various  species  differ  from  one  another  markedly  in 
such  gross  characters  as  color,  weight,  hardness,  chemical  compo- 
sition, width  of  annual  rings,  width  of  rays,  and  number,  size 


Fit;.  62. — Cube  of  pino  wood,  much  enlarged.  ,4,  transverse  section.  B, 
radial  section.  C,  tangential  section.  One  entire  annual  ring  and  parts  of  two 
others  are  shown.      (Courtesy  United  States  Forest  Products  Laboratory) . 


and  arrangement  of  vessels;  and  in  such  microscopic  features  as 
the  size,  shape,  character,  and  location  of  the  different  classes  of 
wood-elements,  the  type  and  distribution  of  pits,  and  the  various 
markings  on  the  cell-walls.  The  structure  of  two  distinct  and 
important  woods,  those  of  pine  and  of  oak,  are  well  shown  in  their 
transverse,  radial  and  tangential  sections  in  Figs.  63  and  64. 
The  various  details  of  wood  structure  remain  so  constant  that 

Fig.  6.3. — Pine  wood  as  seen  under  the  microscope.  A,  transverse  section. 
The  wood  cells  (tracheids)  are  here  cut  across,  at  right  angles  to  their  length. 
Note  the  thin-walled  cells  in  the  spring  wood  and  the  thick-walled  ones  in  the 
summer  wood.  The  wood  rays  run  at  right  angles  to  the  annual  rings.  B, 
radial  section.  C,  tangential  section.  In  both  these,  the  tracheids  are  cut 
lengthwise.  Note  the  marked  difference  in  appearance  of  the  wood  rays  in  the 
three  sections.  The  large  openings  are  resin-canals.  (Courtesy  United  States 
Forest  Products  Laboratory) . 


THE  STEM  AND  ITS  FUNCTIONS 


109 


110  BOTANY:  PRINCIPLES  AND  PROBLEMS 


THE  STEM  AND  ITS  FUNCTIONS  111 

they  may  often  be  used  to  identify  the  plant  species  from  which  a 
piece  of  wood  has  been  derived.  The  great  diversity  which  wood 
displays,  together  with  its  abundance  and  the  ease  with  which  it 
can  be  manipulated,  have  led  to  its  use  in  numberless  ways,  and 
there  is  consequently  no  other  plant  tissue,  aside  from  those  used 
as  food,  which  is  of  such  great  economic  importance. 

The  Ascent  of  Sap  in  Stems. — We  can  determine  by  experiment 
that  water  and  dissolved  substances  absorbed  by  the  roots  are 
carried  upward  in  the  wood  of  the  stem.  As  to  what  causes  this 
movement,  however,  there  is  still  much  question.  To  explain  the 
ascent  of  water  in  low-growing  herbaceous  plants  might  be  fairly 
simple,  but  the  factors  which  bring  about  the  lifting  of  water  in 
large  quantities  to  the  tops  of  tall  trees,  sometimes  more  than 
three  hundred  feet  above  the  ground,  are  very  hard  to  determine. 
An  upward  osmotic  pull  is  of  course  furnished  by  the  increased 
sap  concentration  in  the  leaf-cells  which  follows  the  water-loss 
therefrom  in  transpiration,  but  even  granting  a  strong  pull  at 
the  leaf,  the  rise  obviously  cannot  be  due  to  simple  "suction" 
or  atmospheric  pressure.  Nor  is  capillarity  probably  concerned 
to  any  great  extent  in  the  process,  for,  although  water  may  be 
lifted  very  high  in  exceedingly  small  capillary  tubes,  its  move- 
ment is  so  slow  under  these  conditions  that  capillarity  certainly 
could  not  provide  the  large  amounts  of  water  which  we  know 
must  ascend  the  trunk  daily.  Root-pressure,  if  it  were  strong 
enough,  might  perhaps  be  important,  but  root-pressure  is  mani- 
fest in  woody  plants  only  during  the  early  spring  and  is  therefore 
lacking  at  the  season  when  transpiration  is  most  active.  It  has 
been  suggested  that  the  living  ray  and  wood  parenchyma  cells 
may  be  concerned  in  the  upward  movement  of  water  in  some  way, 
perhaps  furnishing  a  continuous  series  of  osmotic  pumps.  These 
cells  may  be  of  some  such  service,  but  we  know  that  for  a  con- 
siderable time,  at  any  rate,  water  may  ascend  through  a  stem 
where  all  the  living  cells  have  been  killed.  The  most  plausible 
hypothesis  yet  put  forward  is  based  on  the  very  high  cohesive 
power  exhibited  by  water  under  certain  conditions.  In  very  thin 
water  columns,  such  as  must  occur  in  the  conducting  cells  of  the 


Fig.  64. — Oak  wood  as  seen  under  the  microscope.  A,  transverse  section, 
showing  one  conaplete  annual  ring  and  parts  of  two  others.  Note  the  very  wide 
vessels  in  the  spring  wood,  the  narrow  ones  in  the  fall  wood,  the  wide  wood 
ray,  and  the  many  very  narrow  ones.  B,  radial  section.  C,  tangential  section. 
Note  the  many  differences  in  structure  between  this  wood  and  that  of  pine. 
(Courtesy  United  States  Forest  Products  Laboratory). 


112  BOTANY:  PRINCIPLES  AND  PROBLEMS 

wood,  this  cohesive  power  is  perhaps  so  strong  that  a  pull  at  the 
top — in  this  case  the  osmotic  pull  at  the  leaf — will  lift  the  column 
bodily,  as  a  rope  might  be  lifted.  There  are  certain  objections 
to  this  explanation,  too,  but  they  are  not  as  serious  as  in  the 
other  hypotheses.  Possibly  several  of  the  factors  mentioned 
may  be  concerned  together  in  the  ascent  of  sap.  We  must 
admit  that  this  problem,  like  so  many  others  in  biology,  is  as 
yet  far  from  a  satisfactory  solution. 

The  Translocation  of  Foods. — The  plant  must  possess  means 
not  only  for  insuring  the  passage  of  a  plentiful  supply  of  water  to 
the  leaves  through  the  wood  of  the  stem,  petioles  and  veins,  but 
also  for  transporting  the  product  of  the  leaf's  activity — the  manu- 
factured food  in  the  form  of  carbohydrates,  fats  and  proteins — 
to  any  region  of  the  plant  where  food  is  used  or  stored.  This 
function  of  translocation  is  performed  chiefly  by  the  sieve-tubes 
of  the  bast.  The  movement  of  organic  substances  by  diffusion 
from  cell  to  cell  is  a  comparatively  slow  process,  but  is  the  only 
means  available  in  regions  remote  from  the  vascular  system. 
Movement  of  food  for  long  distances,  as  from  the  leaf  to  the 
storage  regions  of  stem  and  root,  seems  to  take  place  almost 
entirely  in  the  bast.  Here  the  protoplasmic  connections  from 
sieve-tube  to  sieve-tube  through  the  sieve-plates  do  away  with 
the  necessity  for  diffusion  through  a  long  series  of  membranes 
and  thus  facilitate  the  rapid  transfer  of  substances  from  place 
to  place.  This  importance  of  the  bast  has  repeatedly  been 
demonstrated  by  experiments  involving  "ringing"  or  "girdling," 
in  which  there  is  removed  from  around  the  stem  a  continuous 
encircling  strip  of  tissue,  including  all  bark  and  bast.  It  is  a 
matter  of  common  observation  that  a  tree  in  which  the  trunk  has 
been  girdled  in  this  way  will  ultimately  die.  Although  small  in 
amount,  therefore,  and  rather  inconspicuous  when  compared 
with  the  wood,  the  bast  is  a  vitally  necessary  tissue  in  the 
economy  of  the  plant. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

334.  Most  stems  tend  to  be  stout  below  and  more  slender  above. 
Why  is  this,  and  of  what  advantage  is  it  to  the  plant? 

335.  Why  are  young  trees  often  somewhat  spire-shaped  but  old  trees 
of  the  same  species  fiat  or  convex  at  the  top? 


THE  STEM  AND  ITS  FUNCTIONS  113 

336.  Does  the  trunk  of  a  tree  become  relatively  stouter  or  more 
slender,  compared  with  the  rest  of  the  tree,  as  the  tree  grows  larger? 
Explain. 

337.  What  difference  in  method  of  stem-growth  is  responsible  for  the 
differences  in  shape  between  a  spruce  tree  and  an  elm  tree? 

338.  A  group  of  trees  of  the  same  species,  growing  very  close  together, 
will  often  have  approximately  the  same  shape  as  that  of  a  single,  well 
developed  tree.     Explain. 

339.  What  advantages  and  what  disadvantages  does  a  climbing  plant 
have  as  compared  with  an  erect  one? 

340.  What  advantages  and  what  disadvantages  does  a  plant  with  a 
prostrate  stem  have  as  compared  with  an  erect  one? 

341.  What  advantages  and  what  disadvantages  does  an  herbaceous 
plant  have  as  compared  with  a  tree? 

342.  Trees  and  shrubs  have  hard  and  woody  stems,  but  herbs  very 
much  softer  ones.     Explain. 

343.  The  stems  of  submersed  water  plants  are  very  soft  and  weak. 
Explain. 

344.  Give  an  example  of  a  plant  which  is  practically  stemless. 

345.  By  looking  at  a  leafy  branch  which  has  been  freshly  cut  from  a 
tree,  how  can  you  tell  whether  it  has  been  growing  in  a  vertical,  oblique 
or  horizontal  position  there? 

346.  What  do  you  think  is  the  most  important  function  performed 
by  the  bud-scales?     Explain. 

347.  Do  all  the  buds  on  a  tree  unfold  and  grow  every  season? 
Explain. 

348.  Why  is  a  potato  tuber  "morphologically"  a  stem? 

349.  Why  is  it  that  a  woody  twig  obtains  air  for  its  internal  tissues 
through  lenticels  rather  than  through  stomata,  as  does  a  leaf? 

350.  What  is  there  about  the  structure  of  cork  which  makes  it  such 
excellent  material  for  bottle  stoppers? 

351.  What  do  we  mean  in  saying  that  the  cortex  and  pith  are 
"undifferentiated"  tissues? 

352.  What  is  the  advantage  in  having  the  cells  of  the  conducting 
tissues  much  elongated? 


114  BOTANY:  PRINCIPLES  AND  PROBLEMS 

353.  Wood  has  a  "grain"  which  in  general  runs  parallel  to  the  axis 
of  the  tree.     To  what  is  this  grain  due? 

354.  Whj^  does  wood  split  easily  "with  the  grain"  but  not  "against 
the  grain?" 

355.  What  causes  knots  in  wood? 

356.  A  log  otherwise  free  from  knots  often  shows  them  near  its  center. 
Why? 

357.  Which  will  have  more  and  larger  knots  in  its  wood,  a  tree  grown 
in  the  forest  or  one  grown  in  the  open?     Why? 

358.  Why  is  the  wood  of  knots  apt  to  be  harder  than  the  wood  around 
them? 

359.  Which  will  decay  faster  if  exposed  freely  to  the  air,  heart-wood  or 
sap-wood.     Why? 

360.  By  looking  at  the  cut  end  of  a  board,  how  can  you  tell  the 
position  which  this  board  held  with  reference  to  the  center  of  the  log 
from  which  the  board  was  cut? 

361.  How  can  you  tell  whether  a  piece  of  furniture  is  made  of  veneered 
wood  or  not? 

362.  What  two  ways  do  you  know  for  telling  the  age  of  a  twig? 

363.  What  makes  the  annual  rings  in  wood  clearly  distinct  from  one 
another? 

364.  What  often  makes  it  difficult  to  count  the  annual  rings  of  trees 
which  have  grown  in  warm  regions? 

365.  As  a  tree  grows  older,  which  increases  more  rapidly  in  thickness, 
its  heart- wood  or  its  sap-wood?     Explain. 

,  366.  In  most  woody  plants  it  is  only  the  last  year's  growth  of  bast,  or 
at  most  that  of  the  last  few  years,  which  functions  in  translocating  food. 
Explain. 

367.  Of  what  use  are  the  bast-fibers  to  the  plant? 

368.  What  suggestion  can  j^ou  make  as  to  the  function  of  the  com- 
panion-cells in  the  bast? 

369.  How  would  you  prove  that  the  ascending  stream  of  water  travels 
in  the  wood? 

370.  Species  of  trees  differ  markedly  in  the  height  to  which  they  can 
grow.  Can  you  suggest  a  factor  which  may  be  responsible  for  this 
difference? 


THE  STEM  AND  ITS  FUNCTIONS  115 

371.  How  is  it  possible  for  a  tree  which  is  "hollow-hearted"  to  thrive 
and  grow? 

372.  In  tapping  maple  trees  for  sap,  it  is  necessary  to  run  the  tap  into 
the  tree  for  only  a  very  short  distance.     Why? 

373.  Cut  flowers  will  keep  fresh  longer  if  the  cut  ends  of  their  stems 
are  trimmed  off  daily;  if,  after  cutting,  the  ends  are  placed  in  boiling 
water  for  a  moment ;  if  the  water  in  which  the  flowers  stand  is  frequently 
changed,  or  if  a  little  salt  is  added  to  the  water.  Explain  how  it  is  that 
these  various  procedures  tend  to  effect  the  desired  result. 

374.  How  would  you  prove  that  manufactured  food  travels  in  the 
bast? 

375.  Why  will  a  girdled  tree  ultimately  die? 

376.  Why  is  the  chestnut-bark  disease,  which  attacks  only  the  outer 
bark,  cortex  and  bast,  so  fatal  to  chestnut  trees? 

377.  Why  is  a  wire  ring  or  other  tight  metal  band  around  a  tree 
trunk  likely  to  injure  the  tree  severely  in  time? 

378.  If  a  trunk  is  bound  tightly  with  wire,  a  swelling  of  the  tissues 
finally  appears  above  the  wire.     Explain. 

379.  In  propagating  a  plant  by  "layering,"  a  gardener  bends  a  branch 
down  to  the  ground  and  covers  a  portion  of  it  with  earth  in  order  that 
it  may  root  at  that  point.  Roots  will  grow  much  better  if  the  stem  is 
bent  or  twisted  strongly  at  the  point  where  roots  are  desired.     Why? 

380.  In  "Chinese"  layering,  a  ring  of  bark  is  cut  off  around  the  stem, 
as  far  in  as  the  wood,  and  that  part  of  the  stem  is  covered  with  moist 
moss.  Roots  eventually  appear  just  above  the  ring,  but  not  below  it. 
Explain. 

381.  In  order  to  obtain  very  large  fruits  for  exhibition,  growers  some- 
times "ring"  a  fruit-bearing  branch  some  distance  below  where  the 
fruit  is  growing.     Explain  why  this  has  the  desired  result. 

382.  What  foundation  in  fact  is  there  for  the  old  belief  that  by 
driving  nails  into  the  trunk  of  a  plum  or  a  peach  tree,  larger  fruit  will 
be  obtained? 

383.  If  a  tree  is  "girdled"  while  its  leaves  are  out,  will  the  leaves  wilt 
or  not? 

REFERENCE  PROBLEMS 

49.  Give  an  example  of  a  stem  which  has  assumed  some  of  the  functions 
of  a  leaf;  of  a  root. 


116  BOTANY:  PRINCIPLES  AND  PROBLEMS 

50.  Why  does  a  gardener  use  "brush  "  to  support  peas  but  poles  to  support 
beans? 

51.  Do  all  buds  have  scales? 

52.  Give  an  example  of  buds  which  do  not  arise  at  a  node. 

63.  Why  do  apple  and  elm  trunks  make  better  chopping  blocks  than  do 
most  woods  ? 

54.  What  is  the  essential  feature  in  the  manufacture  of  wood-pulp  ? 

55.  Why  is  paper  that  is  made  from  wood-pulp  so  much  less  tough  than 
that  made  from  cotton  or  linen? 

56.  What  is  the  difference  between  a  wood  which  is  ring-porous  and  one 
which  is  diffusely-porousf 

57.  Wliat  is  the  process  of  veneering  wood,  and  what  are  its  advantages 
and  disadvantages? 

58.  What  is  "quartered"  oak  and  why  is  it  more  expensive  than  ordinary 
oak? 

59.  Wliy  can  oak  be  quartered  to  advantage  although  most  woods  cannot 
be? 

60.  Why  cannot  the  ascent  of  water  in  the  stem  of  a  plant  be  explained  on 
the  principle  of  a  suction  pump? 

61.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Node  Cortex  Lenticel 

Xylem  Tracheid  Phyllotaxy 

Phloem  Cambium  Fibro-vascular 


CHAPTER  VII 
METABOLISM 

The  term  mctaholism,  whether  used  of  animals  or  of  plants, 
refers  to  the  entire  series  of  chemical  changes  and  processes 
involved  in  the  activity  of  the  living  organism.  It  may  be 
divided  roughly  into  constructive  and  destructive  metabolism. 
The  former  process  begins,  in  plants,  with  the  production  of 
simple  carbohydrates  by  photosynthesis,  and  is  concerned  with 
the  construction  therefrom  of  the  more  complex  plant  foods  and 
with  their  storage,  their  digestion,  their  assimilation  into  the 
living  protoplasm,  and  the  growth  of  new  tissues  which  they  make 
possible.  The  latter  process  involves  a  breaking  down  of  the 
living  substance  thus  built  up,  with  the  consequent  production 
of  waste  materials  and  a  liberation  of  the  energy  which  is  neces- 
sary for  the  activity  of  the  organism. 

Plant  Foods. — We  have  already  discussed  the  production  of 
glucose  by  photosynthesis.  Glucose  is  the  basic  plant  food  from 
which  are  ultimately  derived  all  others — the  more  complex  carbo- 
hydrates, the  fats,  and  the  proteins — which  support  the  life  of 
animals  and  plants. 

Before  we  inquire  into  the  characteristics  of  these  various 
food  types  and  their  mode  of  origin,  however,  we  should  consider 
just  what  is  implied  by  the  term  food  itself.  A  broad  definition 
would  make  "food"  include  everything  taken  into  the  body  of 
the  organism  which  is  essential  to  its  life  and  continued  activity. 
Water,  carbon  dioxide,  and  the  various  essential  mineral  salts 
would  thus  be  considered  as  the  food  of  plants,  and  indeed  it  is 
the  last  of  these  which  in  ordinary  speech  are  most  commonly 
referred  to  as  "plant  foods."  From  such  a  conception  of  food 
as  this,  however,  has  arisen  a  fallacious  distinction  sometimes 
drawn  between  animals  and  plants,  namely  that  the  former 
require  organic  food  and  the  latter  only  inorganic.  A  more 
strict  and  perhaps  from  our  point  of  view  a  more  useful  employ- 
ment of  the  term  "food"  limits  its  application  to  anything  which 
supplies  either  of  the  two  fundamental  needs  of  the  organism — 

117 


118 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


energy  and  building-materials.  We  may  therefore  define  food 
as  whatever  furnishes  a  supply  of  available  energy  to  an  organism  or 
contributes  materially  to  the  upbuilding  of  its  tissues.  It  is  the  car- 
bohydrates, fats,  and  proteins  which  provide  the  materials  for 
growth,  and  which,  because  of  their  somewhat  unstable  chemical 
composition,  contain  a  supply  of  potential  energy  readily  utilized 
by  the  organism.  These  are  the  true  foods.  The  essential 
mineral  salts,  which  constitute  a  very  small  portion  indeed  of  the 


Formation 

of 
Carbohydrates 
Fats 
Proteins  -i^ 

and 
Protoplasm 


Fig. 


65. — The  organic  food  cycle.     History  of  the  construction  and  disintegra- 
tion of  the  important  organic  substances  found  in  plants. 


plant  body,  are  neither  tissue-builders  (except  in  a  very  minor 
degree)  nor  energy-producers,  and  hence  cannot  strictly  be 
regarded  as  plant  foods  at  all.  Their  importance  lies  rather  in  the 
fact  that  they  are  necessary,  in  minute  quantities,  to  the 
construction  and  successful  functioning  of  protoplasm  itself. 
Together  with  water  and  carbon  dioxide  they  may  appropriately 
be  called  nutrient  materials. 

The  food  of  plants  and  animals  is  essentially  the  same  and  the 
difference  between  the  two  groups,  therefore,  lies  not  in  the 
character  of  the  food  which  they  use  but  in  the  fact  that  green 
plants  are  capable  of  synthesizing  food  from  inorganic  nutrient 


METABOLISM  119 

materials  and  that  animals  are  not.  Given  a  simple  food  like 
glucose,  however,  both  animals  and  plants  are  able  to  construct 
therefrom  an  endless  variety  of  more  complex  foods  and  of  other 
organic  compounds.  There  is  thus  a  constant  circulation  of 
various  materials  through  air  and  soil  and  through  the  bodies  of 
green  plants,  of  animals  and  of  bacteria,  a  process  by  which 
organic  substances  are  continually  being  built  up  and  broken 
down.     This  Organic  Cycle  is  graphically  represented  in  Fig.  65. 

Foods  may  be  divided  into  three  main  classes,  which  we  call 
carbohydrates,  fats,  and  -proteins.  These  food  types  differ  from 
each  other  in  physical  structure  and  chemical  composition  as  well 
as  in  the  parts  which  they  play  in  nutrition. 

A.  Carbohydrates. — Carbohydrates  are  substances  composed 
entirely  of  carbon,  hydrogen,  and  oxygen,  in  which  the  hydrogen 
atoms  are  about  twice  as  numerous  as  those  of  oxygen.  Glucose 
(C6H12O6),  the  product  of  photosynthesis,  is  an  example  of  a  very 
simple  carbohydrate.  To  this  group  of  foods  belong  the  various 
sugars,  starches,  and  celluloses,  which  comprise  the  great  bulk  of 
the  food  of  animals  and  plants.  Carbohydrates  are  the  chief 
source  of  energy  for  all  organisms  and  provide  most  of  the  build- 
ing material  for  the  plant  body. 

The  sugars  are  soluble  carbohydrates.  Three  are  more 
common  in  plants  than  others.  These  are  glucose  or  grape 
sugar,  C6H12O6,  the  direct  product  of  photosynthesis; /nodose  or 
fruit  sugar,  identical  in  chemical  formula  with  glucose  but  differ- 
ing in  the  arrangement  of  its  atoms  and  in  certain  physical  charac- 
teristics; and  sucrose  (cane  sugar  or  beet  sugar),  with  the  formula 
C12H22O11,  and  produced  from  the  simpler  sugars  by  the  removal 
of  a  molecule  of  water,  thus: 

2C6H12O6  —  H2O  =  C12H22O1] 

These  three  types  of  sugar  are  all  common  in  plants,  though  in 
any  particular  species  one  is  usually  more  abundant  than  the 
others.  Glucose  and  fructose  form  the  bulk  of  the  sugar  of 
fruits  and  of  the  nectar  of  flowers,  from  which  honey  is  derived, 
and  are  common  elsewhere,  glucose  probably  occurring  in  every 
living  plant  cell.  Sucrose  is  abundant  in  the  sugar-cane  and 
sugar-beet  and  is  therefore  the  type  of  sugar  with  which  we  are 
most  familiar.  Sugars  are  stored  in  many  plants  as  reserve 
foods,  but  are  also  widely  distributed  throughout  the  plant  body, 
because  of  the  fact  that  those  carbohydrates  which  are  insoluble 


120 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


must  be  converted  into  sugar  before  they  can  be  transported 
from  place  to  place,  or  before  they  can  be  assimilated  into  living 
protoplasm. 

The  starches  are  insoluble  carbohydrates,  derived  from  glucose 
but  much  more  complex  in  their  chemical  composition.  Their 
general  formula  is  (CeHioOs)!!.  Starch  is  produced  from  glucose 
by  the  removal  of  a  molecule  of  water,  thus: 

nC6Hi206  —  nH20  =   (CeHioOs)]! 

The  formation  of  starch  is  confined  to  certain  plastids  in  the  cell. 
These  are  the  chloroplasts,  in  cells  where  photosynthesis  is  going 


^ 


^      0 
0   ^ 


Fig.  66. — Starch-grains  from  various  plants.     A,  potato.     B,  wheat.     C,  bean. 
D,  corn.     E,  gloxinia.     F,  rice. 

on,  and  the  leucoplasts  in  storage  cells.  In  these  plastids  the 
starch  is  laid  down  in  small  grains  which  increase  in  number  and 
size  until  in  storage  tissues  the  entire  cell-cavity  may  become 
filled  with  them.  Starch-grains  usually  display  very  character- 
istic shapes  and  markings  distinctive  of  the  species  by  which  they 
are  produced  (Fig.  66).  Indeed,  it  is  often  possible  by  this  means 
to  identify  with  certainty  the  source  from  which  a  particular 
sample  of  starch  has  been  derived.  A  definite  core  or  hilum, 
often  cracked  and  shrunken,  appears  in  most  cases  in  the  grain 
and  is  usually  surrounded  by  a  series  of  more  or  less  concentric 
rings  or  striations. 

Cellulose  is  a  carbohydrate  even  more   complex  chemically 
and  physically  than  starch  but  with  the  same  basic  formula. 


METABOLISM  121 

(C'eHioOs)!!.  Because  of  its  comparative  indigestibility  cellulose 
is  not  usually  available  as  a  food,  though  in  certain  plants  a  layer 
of  it  is  deposited  on  the  inside  of  the  cell-wall  during  periods  of 
food-storage  and  is  later  absorbed  and  used  by  the  plant.  Such 
reserve  cellulose  is  in  some  cases  an  important  source  of  food. 
Of  far  more  significance  than  its  use  as  a  food,  however,  is  the 
fact  that  cellulose  is  the  material  out  of  which  the  cell-wall,  and 
therefore  the  entire  skeleton  of  the  plant,  is  constructed. 

B.  Fats. — Fats  resemble  carbohydrates  in  being  compo.sed  only 
of  carbon,  hydrogen,  and  oxygen,  but  differ  from  them  in  the 
relative  proportion  of  these  elements.  The  hydrogen  atoms  are 
approximately  twice  as  numerous  as  those  of  carbon,  but  in 
comparison  with  these  two,  the  amount  of  oxygen  is  very  small. 
Three  common  plant  fats  well  illustrate  the  chemical  composi- 
tion of  this  type  of  food.  These  are  palmitin,  C3H5(C02Ci5H3i)3; 
stearin,  C3H5(C02Ci7H35)3;  and  olein,  C3H5(C02Ci7H33)3.  Some 
fats  are  liquid  and  others  solid  at  ordinary  temperatures.  Fats 
are  insoluble  in  water  and  when  moved  about  from  cell  to  cell 
by  diffusion  a  fat  must  therefore  be  broken  down  into  its  simpler 
and  soluble  components,  glycerine  and  a  fatty  acid.  In  nature, 
fats  are  readily  converted  into  sugars  and  sugars  into  fats. 
Fats  are  not  as  common  a  type  of  food  as  carbohydrates  among 
plants,  but  are  nevertheless  of  frequent  occurrence  in  certain 
situations,  particularly  in  seeds  and  other  regions  where  food  in 
concentrated  form  is  advantageous.  They  are  apparently  not 
produced  by  any  special  plastids  but  appear  in  the  form  of  minute 
droplets  in  the  cytoplasm.  Fats  are  chiefly  important  as  sources 
of  energy. 

C.  Proteins. — Proteins  are  composed  not  only  of  the  basic 
carbon,  hydrogen,  and  oxygen  of  the  carbohydrates  and  fats,  but 
include  nitrogen  also,  and  generally  a  small  amount  of  sulphur. 
They  are  exceedingly  variable,  both  in  chemical  composition  and 
in  physical  properties,  and  are  often  very  unstable.  Protein 
molecules  are  large  and  complex,  as  is  illustrated  by  the  calcu- 
lated formulas  of  two  common  plant  proteins,  zein  of  corn,  C736 
H1161N184O208S3;  and  (jliadin  of  wheat,  C685H1068N196O211S5.  We 
know  less  alxnit  prot(Mns  than  a])out  any  other  class  of  organic 
compounds.  They  are  of  particular  interest  from  the  fact  that 
protoplasm  itself  is  a  mixture  of  complex  proteins. 

Proteins  undoubtedly  are  formed  by  the  union  of  a  simple 
carbohydrate,  such  as  the  glucose  which  has  been  produced  in 


122  BOTANY:  PRINCIPLES.  AND  PROBLEMS 

photosynthesis,  with  nitrogen  and  sulphur  which  have  been  taken 
into  the  plant  from  the  soil  through  the  root-hairs.  This  union 
probably  occurs  most  commonly  in  the  leaves.  It  is  a  noteworthy 
fact  that  (with  a  few  exceptions)  plants  alone  seem  to  possess 
the  ability  to  bring  about  this  synthesis  of  protein,  an  ability 
almost  as  significant  to  the  rest  of  the  organic  world  as  is  that 
which  enables  them  to  manufacture  carbohydrates  from  inorganic 
substances.  Animals  depend  almost  entirely  upon  plants  for 
their  supply  of  proteins.  If  given  the  simple  protein  compounds, 
however,  animals  can  build  therefrom  new  and  characteristic 
protein  materials  of  all  kinds. 

Proteins,  with  their  highly  complex  molecules,  are  not  produced 
directly  but  are  built  up  by  an  aggregation  of  simpler  nitrogenous 
compounds,  the  amino-acids,  which  have  been  called  their 
"building-stones".  Over  twenty  of  these  compounds  have  been 
isolated  and  studied.  Among  these  are  glycine,  C2HBNO2; 
leucine,  CeHuNOa;  glutamic  acid,  C5H9NO4;  and  tryptophane, 
C11II12N2O2.  From  these  amino-acids,  with  the  addition  of 
sulphur,  are  constructed  all  the  proteins  in  almost  infinite  variety, 
— the  albumins,  globulins,  glutelins,  prolamins,  and  many  others, 
which  differ  in  composition,  solubility,  stability,  and  other 
respects. 

Most  of  the  protein  which  is  stored  as  a  reserve  food  in  plants 
occurs  in  definite  bodies,  the  aleur one-grains,  which  are  secreted 
by  the  protoplasm  much  as  are  starch-grains  and  which  often 
fill  the  sap-cavity.  The  storage  of  proteins  in  this  form  is  fre- 
quently confined  to  particular  regions  of  the  plant.  Thus  the 
aleurone  layer  of  cereal  grains,  a  single  layer  of  cells  just  under 
the  pericarp,  is  filled  with  aleurone  and  is  thus  rich  in  protein. 

Proteins  are  much  less  abundant  as  reserve  foods  than  are 
carbohydrates,  and  are  relatively  poor  energy-producers,  but 
their  composition  makes  them  far  more  effective  than  any  other 
foods  in  the  construction  and  renewal  of  living  substance.  As 
"tissue-builders"  proteins  therefore  play  a  vital  part  in  the 
nutrition  of  plants  and  animals. 

Digestion. — We  have  already  seen  that  the  physiological  proc- 
esses of  a  plant  are  concerned  almost  entirely  with  substances 
which  are  in  solution,  and  that  in  order  to  enter  the  plant  body 
from  the  soil  or  the  air  or  to  pass  from  one  cell  to  another  within 
it,  a  substance  must  first  be  dissolved.  Insoluble  materials  are  of 
little  significance  in  the  economy  of  protoplasm.     Most  of  the 


METABOLISM  123 

food  substances  which  we  have  discussed  above  exist  commonly 
in  forms  which  are  not  sohible  in  water.  The  advantages  of  this 
in  the  storage  of  reserve  foods  is  obvious.  It  is  evident,  however, 
that  before  such  foods  can  be  moved  or  translocated  within  the 
plant,  and  before  they  can  be  assimilated  into  living  protoplasm, 
they  must  in  some  way  be  made  soluble;  and  it  is  this  process  of 
converting  an  insoluble  food  into  soluble  form  which  is  known 
as  digestion. 

Digestion  is  brought  about  through  the  activity  of  certain 
highly  important  but  little  understood  substances  known  as 
enzymes  or  ferinents.  Enzymes  are  concerned  not  alone  in  diges- 
tion but  in  the  production  of  many  other  chemical  changes  in 
the  plant.  They  occur  in  great  variety  and  are  probably  protein 
in  character,  although  their  composition  is  not  definitely  known. 
Enzymes  are  usually  present  in  exceedingly  small  quantities  but 
are  able  to  effect  profound  chemical  changes  out  of  all  proportion 
to  their  bulk.  How  they  do  this  we  do  not  understand.  The 
enzyme  apparently  does  not  enter  into  the  composition  of  the 
substance  produced,  nor  does  it  contribute  energy  for  the  process, 
and  it  is  not  consumed  or  used  up.  It  seems  merely  to  hasten  a 
chemical  reaction  which  might  still  take  place,  although  very 
slowly,  in  its  absence.  Enzymes  have  thus  been  said  to  "lubri- 
cate "  reactions.  Temperature  largely  controls  their  rate  of  activ- 
ity, each  enzyme  having  an  optimum  temperature  at  which  it 
works  most  rapidly.  These  remarkable  substances  may  be 
destroyed  by  heat  and  even  by  certain  poisons.  Aside  from  effect- 
ing digestion,  they  are  concerned  with  the  changes  which  take 
place  in  the  various  fermentations  and  in  decay;  with  the  process 
of  oxidation  in  living  tissues,  and  with  the  synthesis  and  decom- 
position of  many  organic  substances.  Indeed,  most  of  the 
metabolic  processes  of  plants  and  animals  are  probably  depend- 
ent, in  one  way  or  another,  upon  enzymes. 

It  is  only  the  digestive  enzymes  and  their  activities  with 
which  we  are  here  concerned.  Digestion  is  generally  accom- 
panied by  hydrolysis,  or  the  addition  of  one  or  more  molecules 
of  water  to  a  molecule  of  the  substance  to  be  digested.  The 
sugars  are  soluble  and  most  of  them  may  be  assimilated  directly 
without  digestion.  Cane  sugar,  however,  is  often  broken  down 
into  glucose  and  fructose  through  the  agency  of  the  enzyme 
invertase,  thus: 

C12H22O11  -j-  H2O  =  2C6H12O6 


124  BOTANY:  PRINCIPLES  AND  PROBLEMS 

Maltose  is  also  converted  into  glucose  by  maltase.  Far  more 
important  than  these  changes,  however,  is  the  digestion  of  starch 
through  the  action  of  diastase  and  other  enzymes,  with  the  ulti- 
mate production  of  glucose,  thus: 

CeHioOo  +  H2O  =  C6HX2O6 

The  reserve  celluloses  are  broken  down  by  hydrolysis  into  various 
sugars  through  the  action  of  another  enzyme,  cellulase.  Fats,  by 
the  agency  of  lipase  and  similar  enzymes,  are  broken  into  gly- 
cerine and  fatty  acids,  which  may  be  absorbed  into  protoplasm. 
Protein-digesting  enzymes  are  necessarily  numerous,  but  two 
of  them,  pepsin  and  trypsin,  are  especially  important.  The 
former  converts  proteins  into  water-soluble  peptones  and  pro- 
teoses; the  latter  carries  the  process  still  further  with  the  ultimate 
production  of  amino-acids.  It  should  be  noted  that  all  these 
types  of  digestion  are  carried  on  within  the  protoplasm  of  living 
cells  throughout  the  plant  body  wherever  digestion  is  necessary, 
and  not  in  the  cavities  of  special  digestive  organs. 

Assimilation. — After  a  food  has  been  digested,  it  must  then 
enter  the  protoplasm  of  a  cell  and  become  an  integral  part  of  the 
living  substance.  About  this  process,  which  is  known  as  assimila- 
tion and  which  is  really  the  central  problem  of  metabolism,  we 
know  very  httle.  From  the  activities  of  protoplasm  it  is  clear 
that  this  remarkable  substance  must  be  very  complex,  both 
chemically  and  physically.  It  is  highly  unstable  and  is  continu- 
ally undergoing  processes  of  construction  and  destruction. 
Although  we  can  trace  with  some  confidence  the  entrance  into 
protoplasm  of  certain  comparatively  simple  substances  and  the 
departure  therefrom  of  others  equally  simple,  we  must  plead 
almost  complete  ignorance  as  to  the  happenings  which  take 
place  between  these  two  events.  It  is  here  that  dead  matter 
becomes  alive,  that  inert  food  substances  become  endowed  with 
those  unique  properties  of  protoplasm  which  in  the  aggregate 
we  call  life.  This  change  never  occurs  spontaneously  in  nature, 
but  is  always  brought  about  through  the  activity  of  living 
substance  already  existing.  As  far  as  experience  tells  us,  life 
always  comes  from  life  and  in  no  other  way.  Although  this 
process  is  going  on  continually  in  every  living  plant  and  animal, 
we  have  as  yet  been  quite  unable  to  master  its  intricacies  and  to 
imitate  it  in  the  laboratory. 


METABOLISM  125 

Respiration. — Hitherto  we  have  been  considering  those  physio- 
logical changes  which  involve  the  progressive  building  up  and 
elaboration  of  organic  materials,  a  constructive  process  which 
reaches  its  climax  in  the  production  of  protoplasm.  This 
constant  upbuilding  and  renewal  of  the  living  substance  is  suc- 
ceeded by  an  equally  constant  process  of  disintegration,  which 
results  in  the  liberation  of  energy  and  which  is  usually  accom- 
panied by  the  intake  of  oxygen  and  the  outgo  of  carbon  dioxide. 
To  this  general  process  the  name  respiration  has  been  given. 

Before  we  enter  upon  a  detailed  study  of  this  important  phase 
of  plant  physiology  we  should  discuss  briefly  the  problem  which 
it  brings  up,  namely  the  energy-relations  of  the  plant,  of  which 
the  processes  of  food  synthesis  and  nutrition  form  an  essential 
part.  Like  every  living  thing,  the  plant  is  continually  active. 
This  activity  shows  itself  in  movement  of  various  sorts,  either 
of  the  plant  body  as  a  whole,  of  the  substances  within  it,  of  the 
atoms  and  molecules  during  those  chemical  changes  which  are 
always  taking  place  in  living  cells,  or  in  the  phenomena  of 
growth.  These  various  movements,  the  maintenance  of  which 
is  necessary  if  the  plant  is  to  remain  alive,  require  the  expenditure 
of  energy,  as  do  any  movements  of  matter;  and  one  of  the  chief 
problems  in  the  economy  of  the  plant,  as  in  the  operation  of  a 
machine,  is  to  obtain  an  adequate  supply  of  energy  and  to  liberate 
it  at  the  proper  times  and  in  the  proper  places. 

Kinetic  and  Potential  Energy. — Energy  exists  in  the  universe 
in  two  forms:  Active  or  kinetic  and  stored  or  potential  energy. 
Kinetic  energy  performs  work  by  setting  matter  in  motion, 
sometimes  by  changing  its  position,  sometimes  by  raising  its 
temperature,  sometimes  by  producing  chemical  alterations 
within  it,  and  sometimes  in  other  ways.  Potential  energy  is 
inactive  energy,  stored  up  in  an  object  by  virtue  of  the  position 
or  condition  of  that  object.  Potential  energy  exists  in  a  stretched 
spring,  in  a  bent  bow,  in  the  water  of  a  mountain  stream,  in  a 
charged  battery,  in  a  piece  of  coal,  or  in  an  explosive.  It  is 
present  in  an  object  only  as  the  result  of  the  previous  expenditure 
of  kinetic  energy  upon  that  object.  Care  should  of  course  be  taken 
not  to  confuse  this  "storage"  of  energy  with  the  storage  of  food 
or  any  other  form  of  matter.  The  presence  or  absence  of  a  supply 
of  stored  energy  in  a  given  body  merely  affects  the  relations 
between  its  parts  and  does  not  alter  in  the  least  the  Inilk  of  the 
object  or  the  amount  of  matter  which  it  contains.     We  need 


126  BOTANY:  PRINCIPLES  AND  PROBLEMS 

only  to  remember  that  a  bent  spring  weighs  no  more  than  an 
unbent  one  or  a  charged  battery  than  an  uncharged  one.  Energy 
and  matter  are  fundamentally  distinct. 

Release  of  Stored  Energy. — The  potential  energy  in  an  object 
may  at  any  time,  under  an  appropriate  stimulus,  become  con- 
verted again  into  kinetic  form  and  do  work,  as  when  the  stretched 
spring  moves  the  mechanism  of  a  watch,  the  bow  moves  the 
arrow,  the  falling  water  moves  a  mill  wheel,  the  battery  moves  a 
telegraph  sounder,  the  burning  coal  converts  water  into  steam 
which  moves  a  machine,  or  the  explosive  moves  a  projectile. 
In  all  of  these  cases  the  supply  of  stored  energy  is  finally  ex- 
hausted and  motion  ceases.  In  this  process  of  converting  kinetic 
into  potential  energy  and  back  again,  no  energy  is  gained  or 
lost,  the  total  amount  remaining  constant. 

A  machine  is  anything  which  controls  and  directs  the  expen- 
diture of  energy  so  that  work  of  a  particular  kind  is  done  at  a 
particular  place  and  time.  One  of  its  prime  necessities  is  an 
ample  supply  of  potential  energy,  and  in  the  machines  with  which 
we  are  most  familiar  this  is  available  in  the  form  of  wood,  coal, 
oil,  or  stored  electricity.  The  Hving  organism  resembles  a 
machine  in  the  fact  that  it,  too,  directs  the  expenditure  of  energy, 
and  it  therefore  needs  a  plentiful  supply  of  this  energy  in  potential 
form  which  it  may  liberate,  in  the  process  of  respiration,  at  any 
point  throughout  its  body  for  the  performance  of  its  many 
activities.  The  fuel  which  the  organic  machine  uses  in  this 
process  we  know  as  food,  and  the  potential  energy  within  this 
food  came  originally  from  the  kinetic  energy  of  sunlight  and  was 
converted  into  potential  form  by  photosynthesis  in  the  green 
cells  of  the  leaf.  Food  resembles  wood,  coal,  or  oil  in  being  a 
sonicwhat  unstable  chemical  compound  which,  through  the  addi- 
tion of  oxygen,  will  rapidly  break  down  and  resolve  itself  into 
simpler  components,  usually  carbon  dioxide  and  water,  and  thus 
release  the  potential  energy  it  contains.  This  process  of  oxidation 
is  common  in  nature.  In  ordinary  fuels  it  takes  place  only  at 
high  temperatures  and  is  then  known  as  combustion.  In  living 
organisms  it  can  go  on  at  much  lower  temperatures  and  is  here 
known  as  physiological  combustion  or  respiration.  In  their 
essential  feature^ — the  liberation  of  energy  in  kinetic  form  by  the 
breaking  down  of  complex  and  unstable  chemical  compounds 
into  simpler  ones  through  the  addition  of  oxygen — respiration 
and  combustion  are  precisely  similar. 


METABOLISM 


127 


Corbon  Dioxide 


Water 


Process  of  Pho1"OSunthcsis 


f(^)mmmmm(c) — > 


I  ^ —   ^ 


Process  of  Respiraiion 


^ 


I  ^ 


Fig.  67. — The  energy  relations  of  photosynthesis  and  respiration.  Diagram 
showing  the  operation  of  a  simple  laboratory  model.  A,  six  molecules  of  carbon 
dioxide  and  six  of  water,  the  raw  materials  for  photosynthesis.  B,  photosynthe- 
sis. By  the  kinetic  energy  of  light,  the  molecules  of  carbon  dioxide  are  broken 
up  and  the  carbon  atoms  are  being  pulled  over  and  attached  to  the  water  mole- 
cules, thus  stretching  the  six  springs  which  unite  the  carbon  and  oxygen.  C,  the 
products  of  photosynthesis,  a  molecule  of  glucose  and  six  of  oxygen.  The  energy 
exerted  in  photosynthesis  is  now  stored  in  potential  form  in  the  glucose  molecule 
(actually,  of  course,  in  the  stretched  si^rings).  D,  respiration.  The  potential 
energy  in  glucose  is  being  released  in  kinetic  form  (by  the  contraction  of  the 
springs)  in  respiration,  as  the  result  of  which  the  atoms  are  again  arranged  as  six 
molecules  of  carbon  dioxide  and  six  of  water.  This,  of  course,  is  a  very  crude 
imitation  of  the  processes  involved  and  should  not  be  interpreted  too  literally. 


128  BOTANY:  PRINCIPLES  AND  PROBLEMS 

A  machine  is  supplied  with  fuel  (its  "food")  by  an  operator, 
and  an  animal  obtains  its  food  by  seizure,  but  the  food  which  a  green 
plant  consumes  must  be  secured  through  the  plant's  own  activi- 
ties. The  significance  of  the  food-making  process  we  call  photo- 
synthesis is  now  more  evident  than  before.  The  kinetic  energy 
which  the  plant,  through  its  chlorophyll,  absorbs  directly  from  the 
rays  of  the  sun  is  here  used  to  do  the  work  of  pulling  together 
carbon  dioxide  and  water  and  uniting  them  into  the  simple  food 
glucose  (Fig.  67).  The  energy  expended  in  accomplishing  this 
union  is  obviously  stored  up  in  potential  form  in  the  molecules 
of  the  glucose  and  of  the  other  foods  or  plant  materials  which  may 
be  derived  therefrom,  just  as  the  energj^  expended  in  winding  a 
clock  is  stored  up  in  the  compressed  mainspring.  Under  appro- 
priate conditions  this  potential  energy  may  be  liberated  anywhere 
and  at  any  time  to  do  work  in  any  organism.  One  of  the  most 
important  principles  of  physiology  is  thus  brought  out — that  food 
is  merely  the  medium  by  which  energy  received  from  the  sun 
intermittently  and  only  in  certain  exposed  organs  is  stored  up, 
carried  to  all  parts  of  the  plant,  and  made  available  for  work  at 
all  times  and  in  all  places.  This  conception  has  been  concisely 
formulated  in  the  metaphor  that  "food  is  a  'storage  battery,' 
charged  in  green  leaves  by  the  sun  and  discharged  in  the  body  by 
respiration". 

The  Importance  of  Photosynthesis. — The  importance  of  photo- 
synthesis to  the  organic  world  lies  in  the  fact  that  this  process  is 
practically  the  only  means  whereby  living  things  can  store  up 
energy.  Green  plants,  the  ultimate  providers  of  all  foods,  are 
the  sole  agencies  through  which  the  animals  and  man  can  tap  the 
abundant  supplies  of  energy  in  the  universe  and  obtain  therefrom  a 
sufficient  quantity  to  maintain  their  varied  activities.  This  energy 
is  all  stored  originally  in  the  molecule  of  glucose,  and  before  it  is 
converted  again  into  kinetic  form  may  pass  through  scores  of 
modifications  and  enter  into  the  composition  of  the  bodies  of  a 
dozen  successive  organisms.  Although  kinetic  energy  is  stream- 
ing upon  the  earth  daily  in  untold  quantities  from  the  sun,  only 
the  chlorophyll-bearing  plants  are  able  to  use  it  directly. 
Even  in  industry,  man  still  depends  very  largely  upon  photo- 
synthesis, since  the  energy  which  he  releases  from  wood,  coal,  and 
oil  was  originally  locked  up  in  these  substances,  in  some  cases 
millions  of  years  ago,  by  the  photosynthetic  activity  of  green 
plants. 


METABOLISM  129 

Respiration  and  Life. — With  an  understanding  of  the  signifi- 
cance of  the  plant's  energy  relationships  and  the  part  which 
respiration  plays  therein,  we  may  pass  to  a  more  detailed  study 
of  respiration  itself.  This  process,  unlike  photosynthesis,  is  not 
carried  on  in  particular  organs  and  under  particular  conditions, 
but  is  universal,  taking  place  under  all  conditions  and  in  every 
living  cell.  Respiration,  indeed,  is  believed  to  be  a  necessary 
accompaniment  of  life  itself,  as  might  be  inferred  from  the  fact 
that  living  protoplasm  is  continually  active  and  is  thus  continu- 
ally expending  energy.  Even  in  cells  which  are  dormant  and  show 
no  external  signs  of  life,  respiration,  though  very  feeble,  may  still 
be  detected.  The  amount  of  respiration  which  takes  place  is  a 
rough  index  of  the  activity  of  the  cell,  organ,  or  organism  studied. 

The  liberation  of  energy  is  the  essential  feature  of  respiration 
and  the  addition  of  oxygen  is  its  usual  accompaniment,  particu- 
larly in  the  higher  plants;  but  in  certain  cases  and  under  certain 
conditions,  notably  among  some  of  the  lowest  members  of  the 
plant  kingdom,  respiration  may  be  carried  on  in  the  absence  of 
oxygen.  These  two  types  we  recognize  as  aerobic  and  anaerobic 
respiration.  They  are  so  different  as  to  require  separate 
consideration. 

Aerobic  Respiration. — Aerobic  respiration  is  essentially  an 
oxidation  process.  Free  oxygen  is  added  to  organic  substances 
(chiefly  carbohydrates)  with  the  consequent  breaking  down  of 
the  latter  into  their  original  inorganic  components,  carbon  dioxide 
and  water,  thus: 

CeHisOs  +  6O2  =  6CO2  +  6H2O 

The  oxygen  is  usually  taken  directly  from  the  atmosphere  through 
stomata,  lenticels,  or  other  openings. 

This  process  of  oxidation  is  one  of  those  chemical  changes 
which  are  assisted  by  enzymes,  and  the  oxidizing  enzymes  or 
oxidases  occur  in  every  living  cell.  As  to  whether  it  is  the 
protoplasm  itself  or  unassimilated  food  substance  within  the 
protoplasm  which  is  oxidized,  there  is  some  difference  of  opinion. 
It  is  certain,  however,  that  most  of  the  carbohydrates  and  fats 
which  are  taken  into  the  protoplasm  soon  furnish,  either  directly 
or  indirectly,  the  material  which  is  oxidized  in  respiration. 
Proteins,  although  their  chief  function  is  constructive,  undoubt- 
edly also  contribute  to  the  supply  of  oxidizable  substances. 
Whatever  nitrogenous  waste  material  is  given  off  in  their  dis- 


130  BOTANY:  PRINCIPLES  AND  PROBLEMS 

integration,  however,  must  immediately  be  assimilated  again,  for 
(at  least  among  the  higher  plants)  nitrogenous  compounds  do  not 
appear  among  the  products   of  respiration. 

Carbon  dioxide  is  almost  invariably  a  product  both  of  aerobic 
and  of  anaerobic  respiration.  Indeed,  the  evolution  of  this  gas, 
even  in  minute  amounts,  is  regarded  as  proof  that  the  organism  is 
respiring  and  therefore  alive.  The  amount  of  oxygen  taken  in 
and  the  amount  of  carbon  dioxide  given  off  are  in  the  long  run 
equal,  though  under  certain  conditions  one  may  temporarily 
exceed  the  other. 

Much  of  the  energy  liberated  in  respiration  ultimately  appears 
as  heat,  and  a  respiring  organism  will  therefore  tend  to  raise  the 
temperature  of  its  surroundings. 

By  comparing  the  chemical  equations  for  photosynthesis  and 
aerobic  respiration,  it  will  be  seen  that  one  is  the  precise  reverse 
or  reciprocal  of  the  other.  Photosynthesis  adds  carbon  dioxide  to 
water  and  produces  sugar  and  oxygen.  Respiration  adds  oxygen 
to  sugar  and  produces  carbon  dioxide  and  water.  These  two 
processes  may  actually  be  going  on  in  the  same  tissue  at  the 
same  time,  a  circumstance  which  has  made  the  study  of  plant 
metabohsm  pecuharly  difficult,  since  one  activity  may  mask  the 
other.  Photosynthesis,  however,  is  confined  to  the  chlorophyll- 
bearing  cells  and  occurs  in  them  only  in  the  presence  of  hght. 
In  such  cells  there  is  a  preponderance  of  photosynthesis  in  the 
daytime  and  of  respiration  at  night.  For  a  brief  period  in  the 
morning  and  again  at  night,  and  for  longer  times  when  illumina- 
tion is  low  or  other  conditions  unfavorable  for  photosynthesis, 
the  two  processes  may  balance  each  other  exactly,  the  tissues 
giving  off  in  photosynthesis  just  the  amount  of  oxygen  which  is 
necessary  to  carry  on  their  respiratory  activity.  Respiration, 
unlike  photosynthesis,  occurs  in  every  living  cell. 

Comparison  between  Photosynthesis  and  Aerobic  Respiration. — A 
brief  comparison  between  photosynthesis  and  respiration  is 
presented  in  tabular  form  below : 

PHOTOSYNTHESIS  RESPIRATION 

Stores  energy  Releases  energy 

Absorbs  carbon  dioxide  Liberates  carbon  dioxide 

Liberates  oxygen  Absorbs  oxygen 

Takes  place  only  in  green  plants  Takes  place  in  all  plants  and  animals 

Takes  place  only  in  chlorophyll-bear-     Takes  place  in  all  living  cells 

ing  cells 

Constructs  food  Destroys  food 

Increases  weight  Decreases  weight. 


METABOLISM  131 

In  its  essential  characteristics — intake  of  oxygen,  liberation  of 
energy  and  raising  of  temperature — the  aerobic  respiration  of 
plants  is  exactly  comparable  to  the  respiration  of  animals,  a  fact 
which  the  complexities  of  plant  metabolism  sometimes  obscure. 

Anaerobic  Respiration  {Fermentation). — Anaerobic  respiration, 
in  which  the  breaking  down  of  chemical  compounds  and  the 
consequent  liberation  of  energy  is  not  accompanied  by  an  intake 
of  free  oxygen,  is  characteristic  of  certain  lowly  plants  and  some- 
times of  higher  ones  when  temporarily  deprived  of  oxygen.  The 
best  known  examples  of  this  process  are  alcoholic  fermentations 
and  alhed  phenomena.  These  were  named  "fermentations" 
from  the  fact  that  the  activity  of  ferments  (now  more  commonly 
called  enzymes)  was  here  first  clearly  shown.  There  is  conse- 
quently a  confusion  in  the  application  of  the  term  fermentation, 
the  commonest  usage  regarding  it  as  practically  synonymous 
with  anaerobic  respiration,  the  other  expanding  it  to  cover  all 
activity  brought  about  by  the  agency  of  enzymes. 

The  most  important  characteristic  of  anaerobic  respiration 
is  the  fact  that  it  does  not  lead  to  a  complete  breaking  down  of  the 
organic  substance  or  food  but  only  to  its  partial  decomposition, 
with  the  result  that  a  quantity  of  more  or  less  complex  by- 
products is  formed  which  still  contain  a  considerable  amount  of 
potential  energy.  The  accumulation  of  these  by-products  often 
stops  the  process  itself. 

The  fermentation  of  sugar  by  yeast  is  the  classic  example  of 
anaerobic  respiration.  The  yeast  plants — minute,  single-celled 
organisms — thrive  in  rather  weak  solutions  of  sugar.  Those 
which  have  easy  access  to  the  air  usually  respire  aerobically,  but 
if  the  supply  of  free  oxygen  is  limited  (as  is  the  case  anywhere 
below  the  surface  of  the  liquid)  or  if  it  becomes  exhausted,  the 
yeast  respires  anaerobically.  The  cells  now  obtain  their  energy 
through  a  partial  decomposition  of  the  sugar,  with  the  formation 
of  carbon  dioxide  and  a  complex  by-product,  ethyl  alcohol,  thus: 

CeHioOe  =  2CO2  +  2C2H5OH  (alcohol) 

This  change  is  effected  by  the  activity  of  an  enzyme,  zymase, 
which  is  secreted  by  the  yeast  cells.  It  is  evident  that  only  a 
portion  of  the  potential  energy  in  the  sugar  has  been  liberated, 
for  the  resulting  alcohol  may  be  absorbed  by  another  organism, 
or  may  be  burned,  and  will  then  yield  a  considerable  amount  of 
additional  energy.    When  the  concentration  of  alcohol  has  reached 


132 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


a   certain   point,  it  poisons  the  yeast    plant  and  fermentation 
ceases. 

The  respiration  of  other  minute  plants  may  also  bring  about 
alcoholic  fermentation,  and  still  others  produce  fermentations 


^^      ^)>AbsorpHon 


Growth 


Fig.  68. — Important  structures  and  functions  of  the  plant.  Diagrammatic 
representation  of  roots,  stem,  leaves,  and  buds  and  their  functions.  Wood  solid 
black,  bast  dotted,  pith  and  cortex  white.  Arrows  indicate  movement  of  mate- 
rials into  the  plant,  along  the  stem,  and  to  and  from  the  leaves.  One  leaf  repre- 
sented in  section.  Water  passes  from  the  root  upward  to  stem  and  leaves, 
through  the  wood.  Manufactured  food  passes  from  the  leaves  both  upward 
and  downward,  through  the  bast.  The  various  gas-exchanges  between  leaf  and 
atmosphere  are  graphically  represented. 


METABOLISM  133 

of  different  types,  such  as  those  having  for  their  by-products 
butyric  acid  (in  the  spoihng  of  butter),  lactic  acid  (in  the  souring 
of  milk),  and  various  others,  many  of  which  are  of  economic 
importance. 

In  all  these  cases  energy  is  liberated  and  may  be  detected  by 
the  consequent  rise  in  temperature,  which  is  often  more  marked 
than  in  aerobic  respiration. 

Certain  micro-organisms  are  exclusively  anaerobic  and  are 
actually  killed  by  the  presence  of  free  oxygen.  Others,  like  yeast, 
may  respire  either  aerobically  or  anaerobically,  depending  upon 
the  external  conditions. 

The  decay  of  dead  organic  matter  is  due  almost  entirely  to 
the  respiration  of  micro-organisms.  If  the  material  is  exposed 
freely  to  the  air,  bacteria  will  break  it  down  rapidly  and  com- 
pletely by  their  aerobic  respiration.  If  free  oxygen  is  unavailable, 
as  is  the  case  within  a  large  mass  of  dead  organic  material,  the 
process  is  carried  on  anaerobically  and  is  slower  and  more 
complicated.  Here  a  whole  series  of  micro-organisms,  each 
specific  in  its  activity,  are  successively  concerned.  One  type  will 
break  down  the  organic  substance  partially,  extracting  a  certain 
amount  of  the  potential  energy  which  it  contains.  In  its  altered 
chemical  state  and  with  its  diminished  supply  of  energy,  the 
remaining  material  is  now  seized  upon  by  another  type  of  micro- 
organism and,  entering  into  the  anaerobic  respiration  of  this 
form,  is  still  further  broken  down  and  loses  still  more  of  its 
potential  energy.  This  process  continues  until  finally  the  whole 
of  the  material  (except  its  mineral  constituents)  passes  into  the 
atmosphere  as  carbon  dioxide,  water  and  nitrogen,  the  original 
materials  out  of  which  organic  substance  is  constructed. 

We  have  now  completed  a  study  of  the  root,  the  stem,  and  the 
leaf,  and  of  the  functions  of  absorption,  conduction,  photosynthe- 
sis, respiration,  and  transpiration.  These  are  the  main  vegetative 
structures  and  functions  of  the  plant  body,  and  are  graphically 
set  forth  in  Fig.  OS. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

384.  Which  of  the  three  main  types  of  food  makes  up  the  largest 
part  of  the  diet  of  animals  and  man? 

385.  What  advantage  is  it  to  the  plant  to  have  its  carbohydrate  food 
stored  chiefly  in  tlie  form  of  starch  rather  than  chiefly  in  the  form  of 
sugar? 


134  BOTANY:  PRINCIPLES  AND  PROBLEMS 

386.  What  is  it  that  makes  fats  the  most  concentrated  of  foods? 

387.  Why  are  proteins  "far  more  effective  than  any  other  foods  in 
the  construction  and  renewal  of  Hving  substance?" 

388.  Crops  belonging  to  the  Legume  family  usually  contain  more 
protein  than  most  crops.     Explain. 

389.  Where  and  when  in  a  plant  does  digestion  take  place  most 
vigorously? 

390.  What  important  difference  is  there  between  the  process  of  diges- 
tion in  plants  and  that  in  animals? 

391.  Just  how  does  food  which  is  stored  in  the  endosperm  of  a  seed, 
and  which  is  therefore  not  in  the  young  embryo  plant  itself,  become 
available  to  this  young  plant? 

392.  In  what  way  do  the  insects  captured  by  an  insectivorous  plant 
become  available  to  it  as  food? 

393.  Why  do  parsnips  taste  so  much  sweeter  in  the  early  spring  than 
in  the  previous  fall? 

394.  Why  are  vegetables  like  peas  and  sweet  corn  much  sweeter  in 
their  young  and  immature  state  than  when  they  grow  older? 

395.  Maple  sap  is  very  sweet  in  the  spring  but  contains  almost  no 
sugar  in  the  summer.     Explain. 

396.  Hay  harvested  before  its  seed  is  ripe  has  much  more  feed  value 
than  it  has  a  few  weeks  later.     Why? 

397.  Certain  fungi  attack  wood,  their  very  delicate,  thread-like 
branches  penetrating  readily  into  the  hard,  woody  tissues.  How  is  it 
possible  for  them  to  do  this? 

398.  Give  three  examples  from  every-day  life  (aside  from  those 
mentioned  in  the  text)  of  the  conversion  of  kinetic  into  potential  energy 
and  its  subsequent  release  in  kinetic  form  again. 

399.  What  is  the  ultimate  source  of  all  energy  liberated  in  the  bodies 
of  plants  and  animals?     Explain. 

400.  Just  where  in  the  plant  body  is  kinetic  energy  changed  into 
potential  energy  and  just  where  is  potential  energy  changed  into  kinetic? 

401.  What  was  the  original  source  of  the  energy  which  we  derive  from 
the  burning  of  wood?     Explain. 

402.  What  main  sources  of  energy  used  by  man  in  his  industries  owe 
their  origin  to  photosynthesis?     What  do  not? 


METABOLISM  135 

403.  Is  photosynthesis  or  respiration  the  more  active  process  in  a 
normal  green  plant?     How  do  you  know  this? 

404.  Why  is  an  excretory  system,  so  necessary  in  animals,  not  needed 
in  the  case  of  plants? 

405.  State  all  the  resemblances  you  can  tliink  of  between  an  organism 
and  the  flame  of  a  candle. 

406.  How  is  it  possible  for  oxygen  to  get  into  a  living  cell? 

407.  How  do  you  think  oxygen  penetrates  to  cells  deeply  seated  in 
the  plant  body? 

408.  Through  what  structures  does  oxygen  enter  (1)  a  young,  grow- 
ing root,  (2)  a  leaf,  (3)  a  woody  twig,  and  (4)  an  old  trunk? 

409.  How  do  submersed  water  plants  get  their  supply  of  oxygen? 

410.  Plants  which  live  in  bogs  or  very  wet  places  usually  have  large 
air  chambers  in  their  tissues,  particularly  in  roots  or  other  subterranean 
parts.     Explain. 

411.  Do  you  think  that  growing  plants  are  good  things  to  have  in  a 
sick-room?     Explain. 

412.  If  a  plant  were  to  be  grown  in  air  which  had  been  freed  of  oxygen, 
would  it  live  longer  in  darkness  or  in  light?     Why? 

413.  A  seedling  plant  which  has  sprouted  and  grown  in  a  dark  place 
will  have  a  dry  weight  which  is  less  than  that  of  the  seed  from  which 
it  grew,  although  the  bulk  of  the  seedling  is  far  greater  than  that  of  the 
seed.     Explain. 

414.  Does  the  increase  in  the  dry  weight  of  a  plant  measure  the 
amount  of  photosynthesis  which  has  taken  place  in  it?     Explain. 

415.  Which  of  three  identical  wooden  posts  will  remain  sound  longest: 
One  left  freely  exposed  to  the  air;  one  driven  into  the  soil  (as  a  fence 
post),  or  one  driven  under  water  (as  a  pile)?     Explain. 

416.  Most  of  the  fossils  of  animals  and  plants  which  have  come  down 
to  us  were  preserved  in  swamps  rather  than  on  high  ground.     Explain. 

417.  Why  is  it  necessary  to  change  the  water  in  an  aquarium  fre- 
quently if  animals  are  living  in  it  alone,  but  infrequently,  if  at  all,  when 
green  plants  are  living  in  it  alone? 

418.  Where  in  a  i)lant  will  you  be  likely  to  find  the  highest  tempera- 
ture? 


136  BOTANY:  PRINCIPLES  AND  PROBLEMS 

419.  The  internal  temperature  of  plants  is  not  far  from  that  of  their 
surroundings;  in  the  higher  animals  it  is  usually  much  above  that  of 
their  surroundings.     Explain. 

420.  Why  do  land  animals  need  to  have  lungs  for  inhaling  and  exhaling 
air,  when  such  structures  are  unnecessary  in  plants? 

421.  Which  will  weigh  more,  a  piece  of  sound  wood  or  a  piece  of 
decayed  wood  of  the  same  volume?  Which  will  yield  more  heat  when 
burned?     Explain. 

422.  Why  do  plants  in  glazed  pots  grow  poorly? 

423.  Why  should  pebbles  or  bits  of  broken  pottery  be  placed  in  the 
bottom  of  a  flower  pot  in  which  a  plant  is  to  be  grown? 

424.  In  propagating  a  plant  by  "cuttings",  a  small  shoot  is  cut  off 
and  the  cut  end  placed  in  damp  soil,  where  it  takes  root.  Sand  is  much 
better  than  a  heavy  clay  soil  for  this  purpose.     Why? 

425.  Cut  flowers  will  keep  longer  in  a  refrigerator  than  at  ordinary 
room  temperatures.     Why? 

426.  In  cranberry  bogs  which  have  been  flooded  in  the  fall  to  protect 
them  from  frost,  the  unripe  berries  suffer  much  more  from  "smothering," 
due  to  lack  of  oxygen  caused  by  the  flooding,  than  do  the  mature,  fully 
ripe  ones.     Why? 

427.  Fermentation  often  generates  a  considerable  gas  pressure,  but 
respiration  does  not.     Explain  this  difference. 

428.  What  causes  yeast  bread  to  "rise"? 

429.  Why  is  it  important  to  boil  down  maple  sap  as  soon  as  it  is  drawn 
from  the  tree  rather  than  to  let  it  stand? 

430.  When  jars  of  preserved  fruit  "spoil,"  why  do  the  covers  some- 
times blow  off? 

431.  A  foundation  of  stable  manure  under  a  hot-bed  will  "heat" 
and  thus  keep  the  soil  warm.     Explain. 

432.  Why  must  the  manure  under  a  hot-bed  be  moist  before  it  will 
"heat?" 

433.  Give  at  least  two  reasons  for  lifting  the  glass  from  a  hot-bed  in 
the  middle  of  a  sunny  day? 

434.  What  danger  is  there  in  putting  insufficiently  dried  hay  into  a 
barn?     Explain. 


METABOLISM  137 

435.  The  danger  mentioned  in  the  previous  question  is  much  less  if 
the  hay  is  sprinkled  Hberally  with  salt  and  if  the  barn  is  very  tightly 
built.     Explain. 

Note. — A  silo  is  a  large,  tank-like  structure,  open  at  the  top.  Green 
and  living  corn  plants,  chopped  up  into  small  pieces,  are  packed  tightly 
into  the  silo  in  the  fall  and  fed  to  cattle  during  the  winter. 

436.  What  prevents  the  contents  of  a  silo  from  decaying? 

437.  During  the  first  few  days  after  a  silo  is  filled,  its  contents  becomes 
distinctly  warm  and  then  gradually  cools  off.     Explain. 

438.  Why  is  it  necessary  to  have  the  walls  of  a  silo  built  very  tightly? 

439.  If  the  contents  of  a  silo  is  not  packed  down  tightly,  it  is  apt 
to  spoil.     Why? 

440.  Why  does  the  upper  layer  in  a  silo  usuallj^  decay? 


REFERENCE  PROBLEMS 

62.  Give  an  example  of  a  plant  rich  in  starch;  in  fat;  in  protein. 

63.  Which  will  produce  more  energy  per  unit  of  weight,  a  carbohydrate 
or  a  fat?     Why? 

64.  Does  fat  play  a  more  important  part  in  animal  or  in  plant  nutrition? 
Explain. 

65.  Why  does  a  starchy  food  keep  better  than  a  fatty  one? 

66.  In  general,  how  are  "organic"  substances  to  be  distinguished  from 
"inorganic"  ones?     Why  were  these  terms  chosen? 

67.  By  wliat  means  does  sugar  become  converted  into  starch? 

68.  Explain  just  what  has  been  the  history  of  a  piece  of  coal  and  why  it 
produces  so  much  energy  when  burned. 

69.  What  non-gaseous  and  unusable  waste  products  sometimes  result 
from  plant  metabolism,  and  what  does  the  plant  do  with  them? 

70.  Do  plants  ever  derive  energy  from  the  oxidation  of  other  compounds 
than  those  of  carbon? 

71.  Give  the  derivation  of  each  of  the  following  terms  and  explain  in  what 
way  it  is  appropriate: 

Metabolism  Enzyme  Respiration 

Digestion  Assimilation  Fermentation 


CHAPTER  VIII 
GROWTH 

We  have  learned  that  food  provides  the  plant  with  the  energy 
needed  to  carry  on  its  various  functions.  A  large  part  of  the 
food  which  the  plant  manufactures  is  therefore  either  broken 
down  directly  by  respiration,  to  liberate  energy  for  immediate 
use,  or  is  stored  up  to  meet  requirements  of  this  sort  which  may 
later  arise.  A  healthy  plant,  however,  produces  more  food  than 
is  necessary  to  maintain  the  activities  of  its  living  substance, 
and  the  surplus  may  be  built  into  the  tissues  and  used  to  pro- 
duce new  protoplasm  and  new  cell  walls,  thus  promoting  the 
growth  of  the  plant  body.  Growth  represents  the  excess  of  con- 
structive over  destructive  metabolism.  A  knowledge  of  just  what 
it  involves,  and  of  just  how  it  takes  place,  is  evidently  necessary 
if  we  are  to  arrive  at  a  clear  understanding  of  the  structure  and 
development  of  the  plant  body. 

The  term  "growth",  in  its  simplest  usage,  refers  to  any 
increase  in  size,  either  of  the  whole  organism  or  of  its  parts. 
This  expansion  may  be  a  mere  swelling  brought  about  by  a  vigor- 
ous absorption  of  water,  or  it  may  be  due  to  an  increase  in  bulk  of 
actual  plant  material — protoplasm  and  the  dead  structures 
secreted  by  it.  All  early  stages  in  growth  are  of  the  former 
type,  and  the  swollen  and  succulent  tissues  thus  produced 
gradually  attain  to  normal  firmness  through  the  deposition  within 
them  of  large  amounts  of  new  material.  Indeed,  early  growth 
may  be  accompanied  by  an  actual,  though  temporary,  decrease 
in  dry  weight. 

In  studying  this  process  of  growth  in  the  plant  as  a  whole  we 
must  remember  that  the  plant  body  is  made  up  of  a  mass  of 
minute  cells.  The  size  of  the  cells  in  any  particular  tissue  and 
within  the  same  species  is  rather  constant,  and  is  believed  to 
approximate  the  size  which  is  most  efficient  for  that  particular 
tissue.  Very  large  cells  and  very  small  cells  would  evidently 
possess  many  disadvantages.  It  is  therefore  clear  that  growth 
must  consist  in  the  production  of  more  cells  rather  than  in  the 

138 


GROWTH 


139 


enlargement  of  those  already  present;  and  the  method  by  which 
new  cells  are  formed  and  added  to  the  plant  body  deserves  care- 
ful study  in  a  consideration  of  the  growth  process. 

The  Production  of  New  Cells. — In  the  previous  discussion  of 
the  plant  cell  (Chapter  IV)  we  noted  that  it  consists  of  a  small 
mass  of  living  substance  or  protoplasm  in  which  two  parts  may 
be  distinguished,  the  undiffer(>ntiated  cijtoplasm  and  the  denser 


Fig.  69.— Cell  division  by  mitosis.  A,  resting  cell,  the  chromatin  of  the 
nucleus  in  a  fine  network.  B,  the  chromatin  is  gathered  into  a  long  thread. 
C,  this  thread  breaks  up  into  chromosomes.  D,  each  chromosome  splits  into  two 
lengthwise.  {B,  C,  and  D  are  called  jirophascs).  E,  metaphase.  The  split 
chromosomes  arrange  themselves  in  a  plane  across  the  equator  of  the  cell,  and  the 
spindle,  with  its  two  poles,  is  formed.  F,  ariapha^ie.  The  chromosome  halves 
separate,  one  complete  set  (eight  in  this  case)  going  to  one  pole  and  the  other  set 
to  the  other  pole.  G,  telophase.  Each  new  group  of  chromosomes  arranges 
itself  into  a  thread  and  a  new  cell  wall  begins  to  appear  between  the  groups. 
H,  two  complete  new  cells,  each  with  a  nuclear  content  equal  and  similar  to  that 
of  A. 

and  more  or  less  spherical  nucleus.  About  the  whole  is  a  cellulose 
wall,  deposited  by  the  living  substance  within  much  as  a  clam- 
shell is  deposited  by  the  living  clam.     In  growing  tissues  where 


140 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


cell  multiplication  is  rapidly  going  on,  the  wall  is  very  thin,  and 
the  sap  cavity,  so  conspicuous  in  mature  cells,  is  absent.  In  the 
formation  of  new  cells  in  such  growing  regions  three  main  stages 
may  be  distinguished:  Cell  division,  in  which  the  number  of 
cells  is  increased  by  the  division  of  each  parent  cell  into  two; 
cell  enlargement,  in  which  these  new  cells  expand  rapidly  to  their 


E  F  & 

Fig.  70. — Diagram  of  mitosis  in  an  ordinary  body-cell.  A,  resting  nucleus. 
B  and  C,  prophases.  D,  metaphase.  E,  anaphase.  F,  telophase.  G,  new  cells. 
The  separate  chromosomes,  each  of  which  has  an  individuality  of  its  own,  are 
differently  marked.  It  is  evident  that  the  chromatic  material  is  divided  exactly 
evenly  between  the  two  daughter  cells.     (Modified  from  Sharp). 


final  size,  and  cell  maturation,  in  which  they  assume  their  mature 
structure  and  characteristics  (Fig.  71). 

Cell  Division. — Cell  division,  technically  known  as  rnitosis, 
is  not  in  most  cases  a  simple  splitting  into  two  of  the  mother  cells 
but  is  accomplished  in  a  rather  complex  manner  (Figs.  69  and  70). 
The  most  active  part  in  the  process  is  played  by  the  nucleus. 
Within  this  body  is  a  characteristic  granular  material  which  stains 
very  deeply  with  certain  dyes  used  in  microscopical  work  and  has 
hence  received  the  name  of  chromatin.  In  an  ordinary  mature 
cell  the  chromatin  is  arranged  in  a  fine  network,  but  when  the 
cell  is  preparing  to  divide,  the  elements  of  the  chromatin  net- 
work come  together  into  thread-like  or  rod-like  masses,  the 
chromosomes.  .The  number  of  chromosomes  is  the  same  in  every 
vegetative  cell  of  the  plant  and  is  constant  for  any  particular 
species.  Thus  in  Indian  corn  the  chromosome  number  is  20, 
in  wheat  16,  in  peas  14,  and  in  tobacco  48.     Soon  after  the 


GROWTH  141 

chromosomes  become  evident,  two  poles  or  apparent  centers 
of  attraction,  arise  in  the  cytoplasm  on  opposite  sides  of  the 
nucleus,  and  from  each  of  these  poles  a  spindle  of  delicate  fibers 
radiates  inward  toward  the  nucleus.  The  nuclear  membrane 
now  disappears  and  the  chromosomes  become  grouped  in  a  plane 
or  plate  stretching  across  the  center  of  the  cell  at  right  angles  to 
the  two  series  of  spindle  fibers,  which  soon  meet  and  form  on(» 
continuous  spindle  reaching  from  pole  to  pole.  At  this  point  each 
chromosome  has  become  split  lengthwise  into  two  daughter 
chromosomes,  one  of  which  now  moves  toward  one  pole  and  the 
other  toward  the  other  pole.  The  whole  chromatin  mass  is 
thus  divided  exactly  into  two  and  the  halves  separate  widely. 
Each  chromosome  group  now  becomes  broken  up  again  into  a 
network  around  which  a  new  membrane  forms,  and  two  complete 
nuclei  are  thus  produced.  Meanwhile  at  the  central  point  of 
each  spindle  fiber  appears  a  thickening,  and  these  thickenings 
soon  enlarge  and  unite  to  form  a  disc  or  plate  across  the  cell. 
Along  this  plate  a  new  cell-wall  is  laid  down,  which  completes 
the  division  of  the  mother  cell  into  two  similar  daughter  cells. 
Why  such  a  complicated  process  as  mitosis  should  be  necessary 
in  cell  multiplication  we  do  not  understand,  but  it  is  perhaps 
concerned  with  the  need  for  making  an  exactly  equal  division  of 
the  chromatin  material,  since  this  part  of  the  nucleus  is  known 
to  be  of  great  importance  in  directing  the  growth  and  differen- 
tiation of  the  organism. 

Cell  Enlargement. — Although  two  new  cells  have  now  been 
formed,  they  still  occupy  together  a  space  no  larger  than  the 
size  of  the  original  mother  cell,  so  that  no  growth  has  as  yet 
really  taken  place.  The  abundance  of  sugar  and  other  dissolved 
foods,  however,  with  which  a  growing  region  is  always  supplied, 
causes  a  high  osmotic  concentration  in  these  young  cells,  and 
water  is  therefore  vigorously  drawn  into  them  by  osmosis.  Since 
the  newly-formed  walls  are  very  thin  and  elastic  they  stretch 
readily,  and  the  small  cells  thus  increase  markedly  in  size  until 
their  permanent  bulk  is  attained.  During  this  rapid  expansion 
the  amount  of  protoplasm  does  not  increase;  and  although  it 
fills  the  whole  of  the  young  cell  it  is  necessarily  restricted  in  these 
larger  ones  to  a  thin  sac  which  lines  the  wall.  The  bulk  of  the  cell 
is  now  occupied  by  the  vacuole  or  sap-cavity  so  characteristic 
of  mature  plant  cells  in  general.  This  process  of  enlargement 
by  absorption  of  water,  following  the  production  of  new  cells  by 


142  BOTANY:  PRINCIPLES  AND  PROBLEMS 

mitotic  division,  causes  most  of  the  obvious  increase  which  we 
see  in  the  size  of  plant  parts. 

Cell  Maturation.- — The  new  tissue  thus  formed  is  very  soft  and 
weak,  owing  to  the  thin  walls  of  its  component  cells,  and  the  third 
stage  in  growth,  maturation,  is  brought  about  by  the  transfer  of  an 
abundant  supply  of  food  into  these  newly-formed  parts  and  the 
consequent  construction  therefrom  of  new  living  substance  and  of 
heavier  walls  until  the  cells  have  reached  the  normal,  mature 
condition  characteristic  of  the  particular  tissue  of  which  they 
form  a  part. 

Growing-points  and  Their  Function. — Such,  in  brief,  is  the 
history  of  the  production  of  new  cells  by  which  the  growth  of  the 
plant  body  takes  place.  It  is  evident,  however,  that  in  mature 
plant  tissues,  where  the  cells  are  surrounded  by  thick  and  firm 
walls,  cell  division  is  no  longer  possible.  Such  tissues  are  thus 
really  locked  within  their  own  walls  and  can  grow  no  further. 
Organs  like  the  leaf  and  flower,  which  rapidly  attain  a  rather 
definite  size  beyond  which  growth  no  longer  takes  place,  do  not 
present  this  problem,  for  here  the  organ  develops  from  a  small 
mass  of  growing  tissue,  enlarges  rapidly  throughout  its  whole 
extent  and  reaches  maturity  in  all  its  parts  at  once,  when  growth 
stops.  In  such  organs  as  the  root  and  stem,  however,  where 
growth  continues  more  or  less  indefinitely  and  where  the  great 
bulk  of  the  tissues  are  necessarily  mature  and  functioning,  there 
must  obviously  be  some  way  of  insuring  the  continued  production 
of  new  cells.  This  is  accomplished  through  the  activity  of 
growing-points  or  meristems,  which  are  merely  groups  of  cells 
remaining  in  an  embryonic  and  undifferentiated  condition, 
thin-walled  and  packed  with  protoplasm.  These  groups  of 
permanently  "young"  cells  occupy  regions  where  growth  is  to 
take  place,  as  at  the  tip  of  the  root  or  stem  or  at  the  cambium. 
Such  a  growing-point  may  long  remain  dormant,  but  when  it 
becomes  active,  cell  division  begins  again  within  it.  The  newly 
formed  cells  which  lie  next  to  the  already  mature  tissue  now 
undergo  enlargement  and  become  themselves  mature.  This 
process  does  not  affect  all  of  the  cells  of  the  growing-point, 
however,  for  the  portion  away  from  the  maturing  cells  still 
remains  undifferentiated  and  continues  to  serve  as  a  manu- 
factory of  young  cells  which  are  to  be  added  to  the  tissue.  The 
growing-point  is  thus  a  rather  small  and  inconspicuous  group  of 
cells,  not  increasing  in  bulk  itself,  but  carried  progressively  out- 


GROWTH  143 

ward  on  the  crest  of  the  tissue  which  it  creates.  The  growing- 
point  may  perhaps  be  compared  with  the  coral  animals,  which 
form  only  a  very  thin  layer  at  the  surface  of  the  coral  reef  but 
by  their  activity  build  the  reef  farther  and  farther  outward  and 
are  carried  out  upon  it;  or  it  may  be  compared  to  a  brick-layer 
constantly  adding  bricks  to  the  top  of  a  wall  and  being  carried 
himself  high  in  air  by  the  wall  which  he  has  made. 

This  method  of  growth  at  a  definite  point  or  laj^er,  through 
the  activity  of  a  meristem,  which  is  so  characteristic  of  plant 
tissues  and  so  different  from  that  employed  in  the  growth  of 
animals,  has  certain  consequences  worthy  of  mention.  It  writes 
in  the  body  an  almost  complete  history  of  the  plant's  growth  and 
development,  for  many  of  the  first-formed  tissues  are  still  present 
(unless  lost  through  decay),  buried  in  the  successively  later 
accretions  which  have  been  added  from  time  to  time.  A  careful 
internal  and  external  examination  of  a  tree  trunk,  for  example, 
enables  us  to  tell  almost  exactly  how  tall  and  how  thick  the  tree 
was  at  any  year  in  its  past  histor}^  An  understanding  of  the 
location  and  activity  of  growing-points  is  desirable  in  the  practice 
of  the  various  methods  of  grafting  and  budding,  for  these  are 
Qecessarily  concerned  with  a  manipulation  of  the  meristematic 
regions. 

There  are  two  general  types  of  growing  points  in  most  plants — 
terminal  and  lateral.  The  former,  which  develop  at  the  tips  of 
roots  and  stems,  cause  an  increase  in  the  length  of  these  organs, 
and  through  their  activity  the  stem  grows  tall  and  its  roots  spread 
farther  into  the  soil.  The  latter,  of  which  the  cambium  is  the 
characteristic  example,  forms  a  ring  or  sheath  of  growing  tissue, 
encircling  the  root  and  stem  throughout  their  entire  extent  and 
causing  these  organs  to  increase  in  thickness. 

Terminal  Growing-points. — The  growing  tip  of  a  root  furnishes 
a  good  example  of  a  terminal  growing-point,  and  a  brief  study  of 
this  region  will  perhaps  enable  us  to  understand  more  clearly  just 
how  such  a  type  of  meristem  functions  (Fig.  71).  At  the  very  tip 
of  the  root  is  the  root  cap,  a  body  of  dead  cells  continually  renewed 
from  the  growing-point  within  as  they  are  sloughed  off  by  friction, 
and  which  protects  the  delicate  root  tip  as  it  is  forced  through  the 
soil.  Just  back  of  this  is  the  meristem  itself,  a  relatively  small 
mass  of  tissue  usually  not  more  than  two  or  three  millimeters  in 
length,  and  composed  of  small,  thin-walled  and  richly  protoplas- 
mic cells.     Cell  division  takes  place  in  this  region  and  here  alone. 


144  BOTANY:  PRINCIPLES  AND  PROBLEMS 


Zone  of  C?l( 
Maturation 


Zone  of  Cell 
Enlargement 


Zone  of  Cell 
Division 


Root  Cap 


Fig.  71. — Longitudinal  section  of  a  growing  root.     In  the  zone  of  cell  division 
the  cells  are  small,  rich  in  cytoplasm  and  rapidly  dividing  by  mitosis.     In  the 


GROWTH 


Ul 


lichiiul  this  is  the  zone  of  growth  or  cell  enlargement,  where  the 
cells  which  have  been  formed  at  the  growing-point  stretch  and 
elongate.  It  is  in  this  zone  (a  few  millimeters  in  length),  and 
here  only,  that  growth  of  the  root  in  hnigth  takes  place  (Fig.  72) ; 
and  it  is  the  force  exerted  here  by  cell  elongation  which  drives  the 
root  tip  through  the  soil.     Just  back  of  this  region,  in  turn,  is  the 


Fig.  72. — Growth  of  the  root  in  length.  Two  squash  seedlings,  the  one  at  the 
right  a  day  or  two  older  than  the  one  at  the  left.  The  change  in  length  of  the 
zones  between  the  markings,  originally  equidistant,  shows  that  growth  in  length 
takes  place  only  very  near  the  tip  of  the  root. 

zone  of  maturation,  where  the  cells,  now  having  attained  their 
full  size,  assume  their  mature  characteristics.  Here  differentia- 
tion begins,  the  central  cells  developing  into  wood  and  bast, 
those  farther  out  into  cortex,  and  the  outermost  ones  into  epider- 
mal cells  and  root  hairs.  It  should  be  noted  that  the  width  of  the 
young  root  is  determined  by  the  width  of  the  meristem,  and  that 
no  lateral  growth  occurs  here  at  this  time.     The  root  grows 

zone  of  enlargement,  the  cells  are  no  longer  dividing  but  are  rapidly  elongating. 
Vacuoles  or  small  sap-cavities  are  beginning  to  appear  in  the  cytoplasm.  In  the 
zone  of  maturation  the  cells  have  attained  their  final  size,  and  the  center  of  each 
is  now  occupied  by  a  large  sap-cavity,  surrounded  by  a  thin  layer  of  cytoplasm. 
In  this  zone  (only  a  part  of  which  is  shown)  the  cells  are  beginning  to  assume  their 
mature  characteristics  and  differentiation  of  the  tissues  is  taking  place.  A,  B, 
and  C:  cells  from  these  three  zones,  much  enlarged. 
10 


146  BOTANY:  PRINCIPLES  AND  PROBLEMS 

in  thickness  through  the  subsequent  activity  of  a  cambium 
farther  back  along  the  root. 

The  terminal  growing-point  at  the  apex  of  a  stem  resembles 
in  its  essential  features  that  described  for  the  root,  but  the  zones 
in  the  growing  region  are  not  usually  so  clearly  distinguishable 
and  their  combined  length  is  greater.  The  meristem  proper  is  at 
the  very  tip  but  a  certain  amount  of  cell  division  is  going  on 
throughout  the  zone  of  growth,  which  here  may  extend  over  a 
distance  of  several  centimeters. 

Lateral  Growing-point  or  Cambium. — The  lateral  growing- 
point  is  somewhat  more  complicated  than  the  terminal  one  and 
its  activities  are  often  a  little  hard  to  visualize.  The  best 
example  of  such  a  meristem  is  the  cambium  of  the  fibro-vascular 
cylinder  of  root  and  stem,  highly  developed  in  all  typical  woody 
plants.  We  have  seen  in  our  study  of  the  stem  (and  except  for 
the  absence  of  a  pith  the  root  is  essentially  similar)  that  the 
fibro-vascular  tissues  are  arranged  in  a  cylinder,  with  a  ring 
of  wood  inside  and  a  ring  of  bast  outside.  Between  these  two 
rings  is  a  very  thin  layer  of  tissue,  in  its  resting  period  often  only 
one  cell  in  width,  which  is  formed  of  the  same  small,  thin-walled 
and  richly  protoplasmic  cells  which  are  characteristic  of  terminal 
growing-points.  This  is  the  cambium,  and  by  its  activity  the 
fibro-vascular  cylinder,  and  thus  the  whole  stem,  grows  progres- 
sively stouter.  Unlike  the  one-sided  terminal  meristems,  how- 
ever, this  lateral  growing  point  adds  to  the  tissues  07i  both  its 
sides  (Figs.  49  and  50).  When  the  cambium  is  active  and  cell 
division  is  taking  place,  the  new  cells  which  lie  on  the  inner  edge 
of  the  cambium,  next  the  wood,  undergo  a  period  of  enlargement 
and  maturation  and  become  the  outermost  wood  cells;  and  the 
new  cells  which  lie  on  the  outer  edge  of  the  cambium,  next  the 
bast,  similarly  develop  into  the  innermost  bast  cells;  but  a  zone  of 
thin-walled  "embryonic"  tissue  still  remains  between  the  two 
(Fig.  73).  Thus  the  cambium,  never  growing  itself,  continually 
adds  to  the  thickness  of  both  its  adjacent  tissues.  Just  as  the 
terminal  meristem  is  carried  out  by  the  growing  root  or  stem  tip, 
so  the  cambium  ring  is  carried  farther  and  farther  away  from  the 
center  of  the  stem  by  the  growing  wood ;  and  the  bast,  lying  out- 
side the  cambium,  is  also  carried  out,  not  alone  by  this  growth  of 
the  wood  but  by  the  increase  in  its  own  thickness  which  has  taken 
place  at  its  inner  edge  (Fig.  74).  Cambial  activity  may  perhaps 
be  crudely  pictured  by  comparing  it  to  the  growth  of  a  wall 


GROWTH 


147 


of  brick  (the  wood)  surmounted  by  a  coping  of  tile  (the  bast); 
and  by  assuming  that  just  at  the  junction  between  brick  and  tile 
a  bricklayer  (the  cam])ium)  is  able  repeatedly  to  insert  a  brick 
and  a  tile,  the  wall  thus  mounting  upward  and  carrying  an 
ever-thickening  coping  of  tile  on  its  top. 


I  Old  Bas-i- 


Nlew  Basi 


Camb'ium 


i-  Klew  IVoool 


Old  Wood 


Fig.  73. — Cambium  of  pine  stem  actively  producing  new  wood  and  bast. 
The  cambium  is  densely  filled  with  protoplasm.  The  youngest  cells  of  wood  and 
bast,  lying  next  the  cambium,  are  still  small,  but  as  they  grow  older  they  soon 
expand  to  their  mature  size.  The  new  wood-cells  are  still  very  thin-walled  and 
retain  a  lining  of  cytoplasm,  which  later  disappears  entirely. 


As  a  consequence  of  this  method  of  growth,  the  youngest  layers 
of  the  wood  are  the  outermost  and  the  youngest  laj-ers  of  the  bast 
are  the  innermost;  and  the  past  history  of  these  two  tissues,  as  it 
is  preserved  in  their  structure,  should  thus  be  read  in  opposite 
directions  (Fig.  75).  In  woody  plants,  where  growth  in  thickness 
is  considerable  and  continuous,  the  bast,  because  of  its  rather 
delicate  texture  and  the  strain  which  is  put  upon  it,  becomes  much 
stretched  and  crushed  in  its  outer  layers.     These  are  ultimately 


148 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


sloughed  off  with  the  bark,  whereas  the  wood,  with  its  much  firmer 
structure,  remains  unchanged.  Well-marked  growth  rings  are 
developed  in  the  wood  so  that  an  inspection  of  this  tissue  as  seen 


One  Year  Old  Two  Years  Old  Three  Years  Old 

Fig.  74. — Growth  of  a  stem  in  width  (diagrammatic).  Transverse  section 
of  three  progressively  older  stems.  Through  the  activity  of  the'  cambium  a  new 
layer  of  wood  and  of  bast  is  added  each  year.  Pith  and  cortex  dotted,  wood 
plain,  bast  lined. 


Fig.  75. — Diagram  of  a  portion  of  a  cross  section  through  a  six-year-old  twig. 
The  annual  rings  which  are  of  the  same  age,  in  wood  and  bast,  are  shaded  simi- 
larly. (From  Ganong,  "  Textbook  of  Botany",  copyrighted  by  the  Macmillan  Com- 
pany.    Reprinted  by  permission) . 


in  cross  section  makes  it  possible  to  read  with  much  accuracy  the 
age  and  past  history  of  the  plant. 

Cork  Cambium. — Another  lateral  growing-point  of  importance 
is  the  cork  cambium  which  product  the  layers  of  corky  bark. 


GROWTH  149 

This  may  ariso  almost  anywhere  in  the  cortex  or  in  the  old  bast, 
and  develops  on  its  outer  face  a  row  of  corky  cells  which  soon  die 
and  constitute  the  waterproof  layer  characteristic  of  bark  tissue. 

Primary  and  Secondary  Tissues. — The  tissues  laid  down  by  a 
cambium  are  apt  to  be  regular  in  the  arrangement  of  their  cells, 
particularly  in  species  where  all  the  cells  are  of  much  the  same 
size.  This  is  due  to  the  fact  that  each  cambium  cell  has  produced 
a  whole  row  of  wood  cells  within  and  of  bast  cells  without,  and 
that  these  cells  are  naturally  arranged  in  a  straight  line  along 
the  radius  of  the  stem  passing  through  the  cambium  cell  which  is 
their  common  ancestor  (Fig.  73). 

Tissues  produced  by  a  cambium  are  known  collectively  as 
secondary  tissues,  and  they  all  display  in  cross  section  this  rather 
regular  arrangement  of  their  cells.  Primary  tissues  are  those 
which  arise  from  a  terminal  growing- point,  and  in  cross  section 
their  cells  are  apt  to  be  arranged  irregularly.  The  epidermis, 
cortex,  and  pith,  and  the  first  formed  wood  and  bast,  are  all 
primary  in  their  origin.  The  great  bulk  of  the  wood  and  bast  in 
woody  plants,  together  with  the  corky  bark,  is  all  secondary. 

Differentiation. — Growth  is  not  mere  increase  in  size  but  gives 
rise  to  definite  organs  and  organ  systems,  which  are  markedly 
different  in  structure  and  function.  The  factors  which  cause 
and  direct  this  differentiation  of  the  plant  body  as  growth  takes 
place  are  not  understood,  but  we  know  that  the  process  may  be 
somewhat  modified  by  various  factors.  Artificial  removal  of  one 
part  of  the  plant,  such  as  is  brought  about  by  pruning,  will 
stimulate  the  growth  of  the  rest,  and  the  removal  of  one  organ 
or  organ  group  will  often  hasten  the  production  of  more  organs 
of  this  particular  type.  The  development  of  certain  organs  is 
also  dependent  on  the  fulfillment  of  certain  rather  definite  exter- 
nal or  internal  conditions.  In  most  perennial  plants,  for  example, 
the  reproductive  structures  (flowers  and  fruit)  develop  only  after 
the  plant  has  succeeded  in  accumulating  an  ample  supply  of 
reserve  food  from  which  they  may  be  built.  Whatever  stimulates 
very  rank  growth  of  the  vegetative  parts,  such  as  an  abundance 
of  water  and  nitrate  salts,  will  tend  to  retard  the  development  of 
floral  organs,  and  conditions  of  the  opposite  sort,  (providing  a 
sufficient  supply  of  reserve  food  is  at  hand),  will  favor  reproduc- 
tion. The  amount  of  photosynthetic  activity,  governed  chiefly 
by  the  number  of  hours  of  simlight  available  per  tlay,  also  has  an 
important  influence  on  the  appearance  or  suppression  of  the 


150  BOTANY:  PRINCIPLES  AND  PROBLEMS 

reproductive  structures.  By  skillful  manipulation,  the  growth 
and  differentiation  of  the  entire  plant  may  thus  to  a  certain  degree 
be  brought  under  control. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

441.  What  is  the  chief  difference  in  method  of  growth  between 
animals  and  plants? 

442.  With  what  other  important  difference  between  animals  and 
plants  is  this  difference  in  method  of  growth  associated? 

443.  What  disadvantage  would  there  tend  to  be  in  very  large  cells? 
In  very  small  ones? 

444.  What  factors  are  there  which  tend  to  limit  the  size  to  which 
a  tree  can  grow? 

445.  In  what  direction,  or  plane,  with  reference  to  the  rest  of  the 
stem  are  most  of  the  cell  divisions  which  take  place  at  the  terminal  grow- 
ing-point of  the  stem?     at  the  cambium? 

446.  What  difference  in  shape  would  you  expect  to  exist  between  the 
cells  at  a  terminal  growing-point  and  those  at  a  cambium? 

447.  The  zone  in  which  elongation  occurs  in  the  root  tip  is  much 
shorter  than  it  is  in  the  stem  tip.  Of  what  advantage  is  this  fact  to  the 
plant? 

448.  Growing-points  of  plants  are  usually  good  to  eat  and  in  a  few 
cases  are  important  human  foods.     Why  is  this  so? 

449.  The  bark  will  separate  very  easily  from  the  wood  of  a  twig  in 
the  spring  but  usually  at  no  other  season.     Explain. 

450.  If  a  nail  is  driven  into  a  tree  trunk  at  a  point  3  feet  from  the 
ground,  what  position  will  this  nail  occupy  in  the  tree  30  years  later? 
What  evidence  from  observation  have  you  for  your  answer? 

451.  In  just  what  part  of  the  stem  does  growth  of  the  pith  take  place? 
of  the  wood?     of  the  cortex?     of  the  bast?     of  the  epidermis? 

452.  Is  the  pith  in  a  one-year-old  twig  wider  or  narrower  than  it  is 
in  a  20-year-old  branch  grown  from  that  twig? 

453.  Where  would  you  find  the  cortex  in  a  tree  trunk? 

454.  What  important  changes  in  the  size  and  character  of  its  tissues 
take  place  as  a  one-year-old  twig  grows  into  a  20-year-old  branch? 

455.  Why  is  the  bark  of  a  tree  almost  always  rough  and  cracked? 


GROWTH  151 

456.  Whj^  does  the  bark  of  a  tree  never  become  as  thick  as  the  wood? 

457.  If  you  were  to  determine  the  age  of  a  tree  by  counting  the  annual 
rings,  where  in  the  tree  would  you  make  the  count?     Why? 

458.  In  a  cut  stump,  the  rings  are  usually  wider  next  the  pith  than 
they  are  far  out  in  the  trunk.     Why? 

459.  How  can  we  use  the  annual  rings  of  old  tree  trunks  to  study 
past  climatic  conditions?     What  cautions  must  we  observe  in  doing  so? 

Note. — In  grafting,  a  small  twig  (the  scio7i)  which  has  been  cut  from 
one  plant  is  placed  in  close  contact  with  a  branch  of  another  plant 
(the  stock).  This  may  be  done  in  several  ways,  but  in  all  cases  the 
tissues  of  the  stock  and  scion  are  both  cut  open  and  so  placed  together 
that  the  cambium  of  one  touches  the  cambium  of  the  other.  If  the 
operation  is  successfully  performed,  the  stock  and  scion  will  unite  and 
the  latter  will  grow  out  as  a  branch  of  the  former. 

460.  In  the  process  of  grafting,  why  is  it  necessary  for  the  cambial 
layers  of  stock  and  scion  to  be  in  close  contact? 

461.  After  a  graft  has  been  successfully  made,  how  does  water  get 
from  the  tissues  of  the  stock  into  those  of  the  scion? 

462.  Why  is  it  important  to  use  a  very  sharp  knife  in  grafting 
operations? 

463.  In  grafting,  why  is  it  necessary  to  cover  the  cut  surfaces  with  wax 
or  a  similar  substance? 

464.  Plants  which  are  not  rather  closely  related  to  one  another 
cannot  be  grafted  together.  What  explanation  can  you  suggest  for 
this  fact? 

465.  Monocotyledonous  plants  can  almost  never  be  grafted.     Why? 

466.  Nurserymen  sometimes  slit  the  bark  lengthwise  on  strong  and 
rapidly  growing  stems  to  hasten  the  production  of  new  wood.  Why 
does  this  practice  aid  in  producing  the  desired  result? 

Note. — Pruning  is  the  process  by  which  certain  twigs  or  branches 
are  removed  from  cultivated  trees  in  order  to  attain  some  desired  result. 

467.  Why  does  careful  pruning  make  a  tree  more  vigorous  and 
healthy? 

468.  Why  is  pruning  generally  done  in  spring,  fall  or  winter  rather 
than  in  summer? 

469.  How  differently  would  you  prune  a  tree  if  you  desired  fruit 
production  from  the  way  you  would  prune  it  if  you  desired  the  produc- 
tion of  timber? 


152  BOTANY:  PRINCIPLES  AND  PROBLEMS 

470.  When  a  branch  is  cut  from  a  tree,  the  wound  thus  caused  will 
usually  heal  over.     How  does  this  healing  take  i^lace? 

471.  If  a  branch  is  cut  off  very  close  to  the  trunk  it  will  heal  over 
much  more  readily  than  if  a  stump  is  left  projecting  out  some  distance 
beyond  the  trunk.     Why? 

472.  Why  are  rapidly  growing  plants  tenderer  than  slowly  growing 
ones? 

473.  Wliy  do  asparagus  stalks  become  tougher  and  less  desirable  to 
eat  as  they  grow  older? 

474.  What  is  the  best  season  to  train  woody  vines  upon  trellises  and 
arbors  or  to  fix  the  permanent  shape  of  woody  plants  in  other  ways? 
Why? 

475.  Trees  in  exposed  places  are  permanently  bent  in  the  direction 
of  the  prevailing  wind.     Why? 

476.  To  develop  large  blossoms  on  a  chrysanthemum  plant,  growers 
cut  off  all  flower  buds  but  the  terminal  one.  Why  has  this  the  effect 
desired? 

477.  If  tobacco  plants  are  "topped,"  (the  upper  part  of  the  stem, 
including  the  small  leaves  and  the  flower  cluster  being  cut  off  when  it 
has  begun  to  develop)  the  lower  leaves  on  the  stem,  which  are  the  valu- 
able ones  commercially,  will  grow  larger  than  they  otherwise  would. 
Explain. 

478.  Which  do  you  think  will  bear  fruit  first,  a  young  seedling  apple 
tree  or  a  scion  of  the  same  which  has  been  grafted  into  a  large  tree? 
Why? 

479.  Why  does  a  pruning  of  some  of  its  roots  often  cause  a  tree  to 
bear  more  flowers  and  fruit? 

480.  Why  do  many  plants  flower  earlier  if  grown  in  pots  than  if  grown 
in  the  open  soil? 

481.  Why  is  it  that  apple  trees,  and  many  other  northern  fruit  trees, 
sometimes  grow  well  in  warm  climates  but  never  bear  much  fruit  there? 

482.  Why  is  a  moist  season  good  for  forage  crops  but  poor  for  seed 
crops? 

483.  In  most  plants  which  produce  bulbs,  it  generally  takes  several 
years  before  a  plant  raised  from  seed  will  begin  to  flower.     Explain. 

484.  Why  does  an  apple  tree  usually  bear  a  large  crop  only  on  alter- 
nate years? 


GROWTH  153 

485.  Does  it  pay  to  take  good  care  of  an  apple  tree  on  the  years  in 
which  it  does  not  bear  heavily?     Explain. 

486.  By  what  methods  would  you  encourage  a  plant  to  flower? 


REFERENCE  PROBLEMS 

72.  Are  all  the  chromosomes  in  a  plant  cell  exactly  alike? 

73.  Give    an  example  of  a  food  product  which  is  derived  from   a  i)lant 
growing-point. 

74.  What  are  the  differences  between  propagating  plants    by    cutting, 
budding,  and  grafting? 

75.  In  grafted  trees,  docs  the  stock  have  any  effect  on  the  growth   and 
character  of  the  branch  that  develops  from  the  scion? 

76.  How  are  "dwarf"  fruit  trees  produced? 

77.  What  is  meant  by  the  "polarity"  of  a  branch? 

78.  At  what  time  of  the  year  is  it  determined  whether  a  bud  which  is 
forming  on  an  apple  tree  will  be  a  leaf  bud  or  a  flower  bud? 

79.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Mitosis  Meristem  Chromosome 


CHAPTER   IX 
THE  PLANT  AND  ITS  ENVIRONMENT 

The  form  which  a  plant  assumes  and  the  activities  which  it 
carries  on  are  due  to  the  combined  effect  of  two  major  causes. 
These  are,  first,  the  inherent  characteristics  of  the  plant  itself, 
determined  by  the  specific  constitution  of  its  protoplasm  and 
transmitted  from  one  generation  to  another  by  heredity;  and, 
second,  the  surroundings  or  environment  in  which  the  plant 
lives.  Plants  are  so  diverse  and  environments  so  varied  that  the 
relations  which  exist  between  the  one  and  the  other  are  many  and 
complicated.  A  study  of  these  relations  forms  the  subject  matter 
of  the  science  of  Plant  Ecology,  some  of  the  problems  of  which  we 
shall  discuss  briefly  in  this  chapter. 

It  is  evident  that  even  for  the  same  plant,  growing  in  the  same 
spot,  the  conditions  of  light,  temperature,  moisture  and  various 
soil  factors  may  change  radically.  Between  two  plants  in  differ- 
ent places,  environmental  differences  may  be  even  more  marked. 
We  have  already  learned  enough  of  plant  physiology  to  know 
that  these  various  external  factors  may  vitally  affect  the  way  in 
which  the  plant  functions,  and  it  is  therefore  evident  that  if  a 
plant  is  to  thrive  and  maintain  itself,  it  must  be  able  to  modify 
its  form  and  activities  to  meet  this  ever-changing  environment 
successfully.  One  of  the  most  remarkable  facts  of  biology  is 
that  organisms  do  possess,  in  greater  or  less  degree,  this  character- 
istic of  advantageous  regulation  of  structure  and  function  in 
conformity  to  the  changing  external  world.  As  to  what  are  the 
causes  of  these  regulations  there  is  much  difference  of  opinion 
and  no  certain  knowledge.  In  describing  plant  activities,  most  of 
which  contribute  so  obviously  to  the  welfare  of  the  individual, 
we  continually  find  ourselves  using  terms  which  imply  purpose  or 
effort.  It  is  indeed  very  difficult  to  describe  the  facts  of  form  and 
function  in  simple  language  without  tacitly  assuming  that  there 
is  within  the  plant  something  which  directs  and  regulates  its  life 
so  that  it  will  tend  to  do  whatever  is  to  its  own  best  advantage. 
Such  an  assumption  is,  of  course,  quite  contrary  to  the  modern 

154 


THE  PLANT  AND  ITS  ENVIRONMENT  155 

scientific  attitude  as  to  plant  physiology,  which  demands  for 
every  observed  change  a  definite  physical  cause  and  not  a  psy- 
chological one;  and  it  introduces  a  deeper  problem  which  is 
clearly  the  province  of  the  philosopher  rather  than  of  the  botan- 
ist. The  latter  should  content  himself  with  carefully  recording 
all  changes  induced  in  the  plant  by  a  changing  environment,  and 
with  analyzing  as  carefully  as  p9ssiblc  the  factors  which  seem  to 
be  responsible  for  these  changes. 

Stimulus  and  Response. — In  such  a  study  it  is  important  to 
remember  that  the  environmental  forces  do  not  act  on  the  plant 
as  they  would  on  a  lifeless  body — on  a  stone  or  a  drop  of  water, 
for  instance — merely  raising  its  temperature,  illuminating  its 
surface,  pulling  it  down  by  gravity  or  affecting  it  in  other  direct 
ways;  but,  instead,  that  each  of  these  forces  acts  as  a  stimulus 
which  brings  forth  on  the  part  of  the  plant  a  definite  response. 
This  response  may  be  either  a  change  of  function  or  a  change  of 
structure.  To  the  same  stimulus  the  response  of  one  plant  may 
be  very  different  from  that  of  another,  and  the  responses  of 
different  parts  of  the  same  plant  may  also  differ  greatly.  The 
stimulus  is  merely  a  trigger  which  releases  a  response.  Just  what 
a  given  response  shall  be  depends  entirely  upon  the  constitution 
of  the  living  substance  of  the  plant  itself.  This  characteristic 
trait  of  protoplasm  whereby  it  is  continually  reacting  or  respond- 
ing to  stimuli  is  known  as  irritability  and  is  a  distinctive  quality 
of  all  living  things.  In  animals,  protoplasmic  irritability  is 
extraordinarily  developed  in  nervous  tissue,  which  receives  the 
stimuli  and  controls  the  responses  of  the  organism.  In  plants, 
however,  no  nervous  system  has  been  differentiated,  and  although 
some  regions  are  much  more  sensitive  than  others  and  may 
evidently  transmit  the  effects  of  a  stimulus  for  a  considerable 
distance,  it  is  the  protoplasm  of  the  ordinary  cells  which  is 
chiefly  concerned  in  the  many  responses  made  by  the  plant. 

It  should  be  noted  that  although  mature  parts  of  the  plant, 
particularly  those  which  are  soft  in  texture,  are  able  to  change 
their  form  and  position  to  some  extent  through  regulating  the 
turgidity  of  their  cells,  it  is  the  young  and  growing  regions  which 
are  most  sensitive  to  stimuli  and  are  thus  best  able  to  bring  about 
regulatory  changes  of  structure  and  position. 

In  any  discussion  of  the  effect  of  the  environment  we  should 
consider  not  only  the  reaction  of  the  individual  plant  but  should 
also  look  at  the  problem  from  the  historical  viewpoint  and  study 


156 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


those  inherited  characteristics  which  enable  it  to  thrive  in  a 
particular  environment  and  which  have  become  implanted  in  the 
species  dm-ing  the  course  of  its  evolution.  Adjustments  of  this 
kind,  either  of  structure  or  function,  we  usually  speak  of  as 
ada'ptations.  The  natural  adaptations  of  cacti  for  desert  life  and 
of  orchids  for  insect  fertilization  may  be  cited  as  examples. 

We  have  called  attention  to  the  complexity  of  the  environment 
in  which  the  plant  grows.     The  first  step  in  an  analysis  of  the 


55 
.§90 


I-^ 


\ 


/ 


/ 


ll        14-        16        18        20       21       24       76       28        JO       32       34       56       38       40       42 
Degrees  Cenfi.qrade 
Fig.  76. — Growth  in  length  of  maize  shoots  per  hour,  when  exposed  for  twelve 
hours  to  a  constant  temperature.     The  minimum  temperature  for  growth  was  in 
this  case  found  to  be   12°  the  optimum  32°  and  the  maximum  43°.     {After 
Lehenbauer). 

relations  between  this  environment  and  the  plant  is  to  isolate 
the  separate  environmental  factors  and  to  study  the  specific  effect 
of  each.  Of  the  wide  range  of  such  factors  a  few  are  particularly 
important  and  deserve  consideration  here,  notably  temperature, 
-light,  gravity,  moisture,  and  various  chemical  substances,  which 
constitute  the  inorganic  environment ;  and  the  surrounding  plants 
and  animals,  or  the  organic  environment. 

Temperature. — It  is  characteristic  of  all  vital  processes  that 
their  maintenance  is  possible  only  within  a  comparatively  narrow 
range  of  temperatures,  and  temperature  changes  therefore  elicit 


THE  PLANT  AND  ITS  ENVIRONMENT  157 

marked  responses  in  the  activity  of  protoplasm.  In  general, 
active  life  is  possible  for  the  higher  plants  between  0  and  50° 
Centigrade,  although  these  limits  vary  much  from  one  species  to 
another.  The  lowest  temperature  at  which  a  given  plant  can 
continue  to  live  is  known  as  its  minimum  temperature  and  the 
highest  as  its  maximum.  At  some  point  between  these  two  the 
plant  displays  its  greatest  activity  and  this  point  is  known  as  its 
optimum  temperature  (Fig.  76).*  The  various  physiological  proc- 
esses which  go  on  within  the  plant,  such  as  photosynthesis, 
respiration,  and  growth,  each  have  their  minimum,  optimum,  and 
maximum  temperatures  and  these  are  not  necessarily  the  same 
for  the  different  processes. 

Members  of  the  vegetable  kingdom  lack  the  high  and  deli- 
cately maintained  body  temperature  which  is  characteristic  of 
the  higher  animals,  and  the  temperature  of  most  plants  follows 
rather  closely  that  of  their  environment,  absorbing  heat  or  losing 
heat  as  the  environment  becomes  warmer  or  colder.  About 
25  per  cent  of  the  radiant  energy  from  direct  sunlight,  and  a 
much  larger  percentage  in  diffuse  hght,  is  absorbed  by  the  plant. 
Most  of  this  is  converted  into  heat,  only  a  small  fraction  of  the 
energy  being  used  in  photosynthesis.  This  heat  would  often 
raise  the  temperature  of  the  plant  above  the  maximum  if  it  were 
not  largely  expended  in  evaporating  water  from  the  plant  tissues, 
and  the  importance  of  transpiration  as  a  cooling  process  is  thus 
again  to  be  emphasized.  The  body  temperature  of  the  plant 
may  sometimes  fall  a  little  below  that  of  its  immediate  environ- 
ment, owing  to  excessive  radiation  or  evaporation;  or  it  may 
sometimes  rise  noticeably  above  it  owing  to  the  release  of  heat 
during  respiration,  especially  in  regions  where  growth  is  active. 
The  latter  phenomenon  is  particularly  conspicuous  in  the  case  of 
intense  bacterial  action,  for  the  amount  of  heat  liberated  by  the 
vigorous  respiration  of  these  lowly  plants  is  often  sufficient  to 
raise  the  surrounding  temperature  markedly. 

The  problem  of  the  temperature  relations  of  plants  is  further 
complicated  by  the  fact  that  a  plant  may  often  become  accommo- 
dated or  "acclimatized"  to  temperatures  higher  or  lower  than  the 
usual  limits  for  the  species,  if  the  plant  is  brought  into  the  new  en- 
vironment very  gradually.  Thus  plants  which  would  normally 
suffer  from  cold  at  a  given  temperature  may  often  be  made  to 

*  These  terms  maximum,  minimum  and  optimum  are  also  used  for  other 
environmental  factors,  notably  light  and  moisture. 


158  BOTANY:  PRINCIPLES  AND  PROBLEMS 

thrive  under  it  by  lowering  the  temperature  gradually.  This 
resistance  of  plant  tissues  to  heat  and  cold  is  also  dependent  to 
some  degree  on  maturity,  for  young  and  growing  tissues  are 
much  more  susceptible  to  injury  therefrom  than  older  ones.  In 
general,  within  any  particular  species,  the  ability  to  withstand 
high  and  low  temperatures  is  correlated  with  the  amount  of  water 
in  the  cells  and  particularly  in  the  protoplasm  itself.  Where 
water  is  abundant,  resistance  is  low;  where  it  is  scarce,  resis- 
tance is  higher. 

There  are  well-marked  inherited  differences  in  the  temperature 
relations  of  plants.  The  optimum  for  an  alpine  plant  must 
obviously  be  far  lower  than  for  a  native  of  the  tropics.  Certain 
algae  have  their  normal  habitat  in  the  water  of  thermal  springs 
and  others  in  the  frigid  arctic  seas.  Melons  have  a  much  higher 
optimum  than  peas,  and  some  varieties  of  apple,  peach,  and  plum 
are  distinctly  "hardier"  than  other  varieties. 

Light. — We  have  already  noted  the  essential  part  which  light 
plays  in  the  life  of  green  plants  through  its  influence  upon  the 
process  of  photosynthesis.  From  light  rather  than  from  heat, 
electricity  or  other  sources,  the  plant  derives  the  kinetic  energy 
which  it  stores  up  in  potential  form  in  its  food;  and  in  this 
important  capacity  of  an  energy-provider  light  is  therefore  essen- 
tial to  all  plants.  This  influence  is  evidently  an  indirect  one, 
however,  particularly  in  the  case  of  non-green  plants,  for  most  of 
these  thrive  in  the  absence  of  hght  as  long  as  their  food  supply 
holds  out. 

Quite  apart  from  its  indirect  role  in  nutrition,  light  exerts 
certain  direct  effects.  Most  notable  of  these  are  the  growth 
movements  made  by  plant  parts  in  response  to  the  stimulus  of 
illumination.  Not  all  plants  and  not  all  parts  of  plants  respond 
in  the  same  way  to  this  stimulus.  As  a  general  rule  the  stem  will 
turn  toward  the  source  of  light,  the  root  away  from  it,  and  the 
leaf,  by  the  twisting  of  its  petiole,  will  assume  a  position  in  which 
the  broad  surface  of  the  blade  is  at  right  angles  to  the  light  rays 
(Fig.  77).  Any  movement  which  is  a  specific  reaction  to  the 
stimulus  of  light  is  known  as  a  phototropism.  A  normal  stem  is 
therefore  said  to  be  positively  phototropic,  a  root  negatively 
phototropic,  and  a  leaf  neutrally  phototropic  or  diaphototropic. 
Although  the  results  of  phototropism  are  usually  much  more 
noticeable  where  a  plant  receives  its  illumination  from  one  side 
only,  it  plays  an  important  part  in  the  orientation  of  plant 


THE  PLANT  AND  ITS  ENVIRONMENT 


159 


structures    generally.     The    advantageous     character    of    the 
ordinary  phototropic  responses  is  obvious. 

Not  only  does  light  affect  the  position  of  plant  organs  but  it  has 
a  profound  influence  upon  their  structure.  /The  stems  of  green 
plants  grown  in  the  dark  are  usually  slender,  much  elongated  and 
provided   with   but  little   woody  tissue;   and  their  leaves    are 


Fig.  77. — Phototropism.  A  mustard  seedling  growing  with  its  root  in  water. 
This  plant  was  at  first  illuminated  from  all  sides,  but  later  from  only  one  (shown 
by  direction  of  arrows).  Note  that  the  stem  has  bent  toward  the  light  and  the 
root  away  from  it,  and  that  the  leaves  have  taken  up  a  position  at  right  angles  to 
the  light.      (After  Strash urger) . 


greatly  reduced  in  size,  long  petioled  and  undifferentiated  inter- 
nally. Chlorophyll  fails  to  develop  and  the  plant  assumes  a 
pale  yellow  color.  This  general  effect  of  darkness  is  known  as 
etiolation  (Fig.  78)  and  begins  to  show  itself  whenever  the  supply 
of  light  falls  below  the  optimum  either  in  duration  or  intensity. 
If  sufficiently  pronounced,  etiolation  ultimately  results  in  death. 
The  stimulus  of  light  upon  protoplasm  evidently  prevents  the 
abnormal  growth  which  we  see  in  etiolation,  but  how  this  effect 
is  brought  about,  we  do  not  understand. 

Too  little  illumination  is  thus  harmful  to  the  plant,  but 
too  much  may  be  e(iually  so  through  its  to.xic  effect  upon  proto- 
plasm. To  the  blue,  violet,  and  ultra-violet  raj^s  living  substance 
is  particularly  sensitive,  and  in  many  plants  the  position  or 


160 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


structure  of  parts  may  be  so  modified  that  as  small  a  surface  as 
possible  is  exposed  to  light  when  it  becomes  very  bright.     In  some 


Fig.  78. — Etiolation.  The  two  bean  seedlings  are  of  the  same  age  but  the  one 
at  the  right  was  grown  under  normal  illumination,  the  one  at  the  left,  in  darkness. 
Note  the  longer  internodes,  the  paler  color,  and  the  poorer  leaf-development  of 
the  darkened  plant. 

species,  for  example,  the  broad  surface  of  the  blade  is  exposed  to 
light  of  moderate  intensity  but  only  the  edge  to  very  bright 
illumination. 


THE  PLANT  AND  ITS  ENVIRONMENT 


161 


Toward  light,  as  toward  temperature,  plants  display  certain 
inherited  adaptations.  Some  green  plants  are  able  to  thrive  in 
light  of  comparatively  low  intensity  and  are  said  to  be  tolerant 
(Fig.  79),  in  the  sense  that  they  will  tolerate  a  considerable  degree 
of  shade.     Others  will  grow  normally  only  where  the  illumination 


Fig.  79. — Plants  tolerant  of  shade.  These  broad-leaved  plants  of  Viburnum 
are  able  to  thrive  on  the  forest  floor,  where  the  lower  branches  of  the  forest  trees 
have  died  from  lack  of  light. 


is  good,  and  are  said  to  be  intolerant  (of  shade).  Some  are  sensi- 
tive to  intense  light  and  thus  cannot  live  in  the  open,  whereas 
others,  either  through  the  greater  resisting  power  of  their  proto- 
plasm or  through  various  structural  modifications,  are  able  to 
withstand  the  most  brilliant  sunlight. 

Gravity .^ — Gravity  in  its  effect  upon  plants  is  unlike  tempera- 
ture and  light  in  that  it  is  both  continuous  in  action  and  constant 
in  amount.  It  is  a  very  important  factor  in  determining  the 
direction  of  growth  in  plant  parts  and,  like  light,  it  affects 
different  organs  in  different  ways.     Any  reaction  to  the  stimulus 


162 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


of  gravity  is  known  as  a  geotropism  (Fig.  80).  Stems  ordinarily 
tend  to  grow  in  a  direction  opposite  to  the  force  of  gravity  and  are 
thus  negatively  geotropic.  Primary  roots  and  certain  other 
portions  of  the  root  system,  on  the  contrary,  tend  to  grow  directly 
toward  the  center  of  the  earth  and  are  thus  positively  geotropic. 


Fig.  80. — Geotropism.  Four  kernels  of  corn  which  have  germinated  in 
different  positions.  The  young  root  in  every  case  has  grown  downward  and  the 
young  shoot  upward. 

Most  leaves  tend  to  take  up  their  position  at  right  angles  to  the 
force  of  gravity  and  many  lateral  roots  also  grow  in  an  approxi- 
mately horizontal  direction.  Such  organs  are  said  to  be  diageo- 
tropic.  The  advantage  of  these  specific  tropisms  to  the  plant  is 
obvious. 

Plants  differ  considerably  in  their  inherited  adaptations  to  the 
influence  of  gravity.  Stems  of  prostrate  plants  have  lost  their 
negative  geotropism  and  may  even  develop  into  rootstocks  or 
tubers  which  react  toward  gravity  like  roots.  The  responses  of 
flowers  and  fruits  to  this  stimulus  are  also  very  diverse  and  for 
the  most  part  seem  to  be  advantageous  to  the  plant. 

The  mechanism  of  stimulus  and  response  has  been  more  care- 
fully studied  in  the  case  of  geotropism  than  with  other  environ- 
mental factors.  If  very  young  seedlings  in  which  the' root  and 
stem  are  just  appearing  are  fixed  in  any  position  whatever,  the 
young  root  will  invariably  grow  downward  and  the  young  stem 
upward  (Fig.  80).  By  attaching  such  seedlings  to  a  disc  and 
revolving  it  rapidly,  centrifugal  force  may  be  developed  which  is 
greater  than  gravity,  and  in  such  a  case  the  young  plants  orient 


THE  PLANT  AND  ITS  ENVIRONMENT  1G3 

themselves  to  this  new  stimukis,  the  roots  growing  outward  in 
the  direction  of  the  force  and  the  stems  inward,  in  the  opposite 
direction.  The  force  of  gravity  may  also  be  practically  elimi- 
nated, without  such  a  substitution  of  another  and  more  powerful 
force,  if  the  disc  to  which  the  seedlings  are  attached  is  slowly 
revolved  in  a  vertical  plane  by  clockwork,  thus  exposing  all  sides 
of  the  seedling  successively  to  the  stimulus.  Under  these  con- 
ditions, the  root  and  the  stem  continue  to  grow  in  the  direction 
in  which  they  happened  to  start  with  no  reference  at  all  to  gravi- 
tation. The  pull  of  gravity,  or  any  other  force  which  we  may 
substitute  for  it,  must  therefore  be  able  in  some  way  to  stimulate 
the  growing  regions  of  root  and  stem  so  that  growth  occurs  in 
certain  definite  directions.  It  has  been  proven  that  the  very  tip 
of  the  root,  for  a  distance  of  approximately  1  or  2  millimeters  in 
length,  is  the  only  portion  which  is  sensitive  to  the  stimulus.  If 
this  region  is  pointing  downward,  no  bending  of  the  root  will  take 
place,  regardless  of  the  position  of  the  rest  of  the  organ.  If  this 
region  is  horizontal,  however,  the  root  will  bend  downward  until 
the  sensitive  tip  itself  points  downward  again.  The  actual  bend- 
ing never  takes  place  in  the  tip,  however,  but  always  in  the  growth 
zone  some  distance  behind.  As  to  how  the  stimulus  is  perceived 
by  the  tip,  and  how  it  is  transmitted  to  the  zone  of  bending,  are 
problems  about  which  little  is  definitely  known. 

Moisture. — Of  vital  importance  to  every  plant  is  the  main- 
tenance within  it  of  a  sufficient  supply  of  water,  and  we  have 
already  called  attention  to  the  preponderant  role  which  this 
substance  plays  in  plant  functions.  It  comprises  over  ninety 
per  cent  of  most  living  plant  tissues.  Practically  all  of  the 
physiological  processes  of  the  plant  are  carried  on  in  solution 
within  it.  The  transportation  of  substances  from  place  to  place 
through  the  plant  body  is  accomplished  by  their  diffusion  (in 
solution)  from  one  cell  to  another.  Water  is  one  of  the  two 
essential  raw  materials  in  photosynthesis.  The  normal  form  and 
proper  functioning  of  the  softer  plant  tissues  is  maintained  by 
keeping  their  cells  completely  filled  and  turgid  with  water.  It  is 
therefore  to  be  expected  that  the  characteristic  structures  and 
activities  of  plants  should  be  concerned  with  obtaining  and  con- 
serving an  ample  supply  of  this  precious  liquid. 

Definite  movements  and  changes  of  position  with  reference  to 
moisture  are  shown  chiefly  by  roots.  The  root  tip  is  sensitive 
to  variations  in  the  water-content  of  the  soil,  and  will  turn  from  a 


164 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


region  of  low  content  to  one  of  higher.  Such  a  response  is  known 
as  an  hydrotropism  and  results  in  the  pursuit  of  moisture  by  roots 
for  considerable  distances,  notably  when  the  surface  layers  of  the 
soil  are  drying  out  and  the  water  table  is  descending. 

It  is  in  responses  of  structural  change  rather  than  those  of 
movement,  however,  that  the  effect  of  moisture  is  most  often 
manifest.     Plants  which  have  access  to  abundant  water  supply 


Fig.  81. — "Amphibious"  plant.  A  shoot  of  mermaid  weed  (Proserjnnaca 
palustris).  Submersed  leaves  are  finely  divided,  aerial  leaves  undivided,  and 
leaves  between,  intermediate  in  form. 

grow  luxuriantly,  for  the  most  part,  with  large  leaves  and  rather 
soft  tissues.  Cuticle  and  epidermis  are  thin,  woody  tissues  some- 
what weak,  and  parenchyma  abundant.  A  similar  plant  grown 
where  water  is  scanty  becomes  stunted  throughout  and  has  much 
smaller  leaves.  Its  tissues,  particularly  the  epidermis  and  cuticle, 
are  much  tougher,  and  the  woody  elements  stronger  and  more 
abundant.  In  some  instances  even  more  profound  structural 
changes  are  produced.  The  water  buttercup,  to  cite  a  notable 
example,  when  growing  on  the  shore  produces  normal  buttercup 
leaves,  but  when  submersed  in  water  develops  leaves  which  are 
dissected  into  fine  capillary  segments  and  are  thus  particularly 
well  fitted  for  aquatic  life.  Other  "amphibious"  plants  exhibit 
similar  structural  changes  (Fig.  81). 


THE  PLANT  AND  ITS  ENVIRONMENT 


165 


Xerophytes. — Inherited  adaptations  to  abundance  or  dearth 
of  water  show  the  pronounced  effects  of  moisture  as  an  environ- 
mental factor.  Many  plants  have  become  so  modified  during 
the  course  of  evolution  that  they  are  able  to  thrive  under  con- 
ditions where  the  available  soil  water  is  comparatively  small  in 


Epidermal 
Cell 


Fig.  82. — The  stoma  of  a  xerophytic  plant  (Cycas).  The  stoma  is  protected 
by  being  sunken  in  a  pit  formed  by  the  over-arching  growth  of  two  adjacent 
epidermal  cells. 

amount,  and  where  plants  without  special  adaptive  modifications 
would  speedily  perish.  Such  drought-loving  plants  are  known  as 
xerophytes  and  are  characterized  by  several  types  of  structural 
and  functional  modifications  which  result  in  a  notable  ability 


Fig.  83.- — Section  across  the  blade  of  a  xerophytic  leaf  {Empclrum) .  The 
blade  is  rolled  back  so  that  the  margins  almost  meet,  thus  preventing  excessive 
transpiration  from  the  stomata,  which  are  on  the  lower  (inner)  surface.  A  felt 
of  woolly  hairs  is  also  developed  at  the  leaf  margins  which  further  hinders  the  loss 
of  water. 

both  to  draw  water  from  the  soil  and  to  retain  it  in  the  plant 
tissues.  The  root-system  is  very  well  developed  in  proportion 
to  the  shoot.  The  osmotic  concentration  of  the  cell  sap  is  usually 
higher  than  among  plants  growing  under  less  arid  environments. 
The  leaf  surface  is  reduced,  sometimes  very  radically;  and  leaves 
may  even   disappear  entirely.     The   cuticle   of   stem  and  leaf 


166 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fig.  84.-^One  type  ut   xi'iuphjto.     The  ^toiiodop   (Sfdum),  showing  its  very 
thick  and  fleshy  leaves. 


Fig.  85. — Hydrophytes.  The  type  with  broad,  floating  led\e.->  (water  lily) 
and  that  with  small,  delicate  leaves,  mostly  submersed  (water  milfoil),  are  both 
shown. 


THE  PLANT  AND  ITS  ENVIRONMENT  167 

becomes  extraordinarily  thick.  Stomata  are  relatively  few  and  are 
usually  either  sunken  in  pits  (Fig.  82),  covered  with  a  mass  of 
hairs,  or  otherwise  protected  (Fig.  83).  In  certain  cases,  notably 
among  arctic  and  alpine  xerophj^es,  the  leaves  and  stems  are 
covered  with  a  felting  of  hairs.  In  others,  particularly  the 
xerophytes  of  saline  soils,  the  vegetative  organs  become  fleshy 
and  succulent  (Fig.  84),  Internally,  most  xerophytes  have  an 
abundant  development  of  woody  tissue. 


Fig.  86. — Transverse  section  of  the  stem  of  a  typical  water-plant  {Myrio- 
phyllum).  The  fibro-vascular  cylinder  is  poorly  developed  and  the  cortex  is 
provided  with  large  air  chambers. 

Hydrophytes. — At  quite  the  opposite  extreme  from  xerophytes 
are  those  plants  which  are  adapted  to  live  nearly  or  quite  sub- 
mersed in  water.  These  hydrophytes  (Fig.  85)  have  root-systems 
which  are  much  reduced  or  may  even  be  entirely  absent.  The 
leaves,  if  submersed,  are  usually  finely  cut  and  dissected  and  are 
very  thin,  Stomata  are  absent.  The  stems  have  become  very 
soft  and  weak  and  possess  an  exceedingly  small  amount  of  woody 
tissue.  In  certain  types,  notably  those  in  which  the  leaves 
(or  some  of  them)  are  exposed  to  the  air,  the  tissues  of  the  plant 
are  well  provided  with  air  passages  (Fig,  86), 

Mesophytes. — Those  plants  with  which  we  are  most  familiar 
thrive  best  on  a  moderate  supply  of  water  and  are  known  as 
mesophytes.  Living  under  conditions  especiall}'  favorable  for 
plant  growth,  they  possess  well  developed  root  and  leaf  systems 
and  are  generally  large,  thrifty,  and  fast-growing  as  compared 


168  BOTANY:  PRINCIPLES  AND  PROBLEMS 

with  xerophytes  and  hydrophytes.  Their  leaf  area  is  extensive, 
the  cuticle  and  epidermis  rather  thin,  and  the  stomata  not  especi- 
ally protected.  Internally  they  are  highly  differentiated,  partic- 
ularly as  to  the  fibro-vascular  system. 

Through  bog  and  swamp  plants,  with  their  typically  spongy 
internal  structure,  mesophytes  merge  gradually  into  hydrophytes ; 
and  at  the  other  environmental  extreme  it  is  hard  to  draw  a  sharp 
Hne  between  mesophytes  and  xerophytes.  Furthermore,  there 
are  many  plants,  such  as  our  deciduous  trees,  which  are  meso- 
phytic  during  the  summer  and  xerophytic  during  the  winter. 
Such  plants  are  known  as  trophophytes. 

The  modifications  of  plants  with  reference  to  their  water 
supply  are  legion  and  provide  a  fascinating  field  of  investigation, 
for  nowhere  else  is  the  regulatory  character  of  plant  structures 
more  clearly  evident. 

Chemical  Substances. — The  effects  of  chemical  substances  upon 
plants  are  many  and  far-reaching.  In  certain  cases,  particularly 
in  the  lower  groups,  the  direction  of  growth  or  movement  may  be 
determined  by  chemical  stimuli.  Far  more  profound  are  the 
structural  and  functional  changes  induced  by  chemical  substances 
entering  the  plant  from  the  soil. 

We  have  already  enumerated  the  chemical  elements  which  are 
essential  to  the  life  of  plants  and  have  called  attention  to  certain 
of  their  specific  effects.  Iron,  for  example,  has  been  found  to  be 
necessary  for  the  production  of  chlorophyll.  Phosphorus  is 
abundant  in  seeds  and  is  beheved  to  stimulate  the  development  of 
reproductive  structures.  Potassium  seems  to  have  an  intimate 
relation  to  the  process  of  photosynthesis  and  an  ample  supply 
of  this  element  is  necessary  if  the  leaves  are  to  become  well 
developed  and  to  function  vigorously.  Nitrogen,  especially 
when  accompanied  by  an  abundant  water  supply  and  active 
photosynthesis,  markedly  stimulates  the  development  of  vege- 
tative organs  and  tends  to  delay  the  production  of  flowers  and 
fruit.  These  elements  and  others  not  only  affect  the  vigor  and 
size  of  the  plant  but  often  the  shape  and  structure  of  its  parts. 

Chemical  substances  injected  into  the  plant  body  are  also 
able  to  stimulate  growth  in  various  abnormal  ways.  Notable 
instances  of  this  are  the  great  variety  of  galls  produced  on  plant 
tissues  through  insect  stings  or  fungus  attacks  (Fig.  87). 

Plants  display  characteristic  inherited  reactions  to  the  presence 
of  chemical  substances  in  the  soil.     Certain  species,  for  example. 


THE  PLANT  AND  ITS  ENVIRONMENT 


169 


thrive  on  saline  soil  where  the  majority  of  plants  are  quite  unable 
to  exist.  Others  grow  only  where  the  soil  is  markedly  acid, 
and  still  others  only  where  lime  is  abundant.  Some  ubiquitous 
types  are  able  to  exist  in  soils  of  almost  any  chemical  composition. 
The  distribution  of  plant  species  over  the  earth's  surface  is  influ- 
enced to  no  small  degree  by  their  reactions  to  particular  chemical 
substances  in  their  environment. 

A  number  of  species  may  be  all  adapted  in  a  similar  way  to 
these  various  physical  conditions  of  the  environment  which  we 


/ 

SMS* 

^mm 

/ 

^m 

/  < 

i| 

Fig.  87. — Galls  produced  by  the  attack  of  a  fungus  {Gymnosporangium)   on 
red  cedar. 

have  just  been  discussing  and  may  thus  grow  side  by  side  together, 
forming  an  easily  recognizable  group.  Such  a  group  is  known  as 
a  Plant  Association.  We  may  distinguish,  for  example,  a  Swamp 
Association  (Fig.  88),  a  Desert  Association  (Fig.  89),  a  Meso- 
phytic  Forest  Association,  a  Sea  Beach  Association,  and  many 
others,  each  with  its  characteristic  species  and  its  characteristic 
structural  and  functional  modifications.  As  the  environment 
changes,  one  association  may  encroach  upon  another  and  the 
vegetation  of  a  region  may  thus  be  gradually  altered.  A  study 
of  plant  associations  and  their  history  is  an  important  part  of  the 
science  of  ecology. 


170 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fig.  88.- — A  Swamp  Association.      Note  tlic  contrast  between  the  plants  which 
compose  this  Plant  Association  and  those  in  Fig.  89. 


Fig.  89. — A  Desert  Association,  composed  of  cacti  and  other  extreme  xero- 
phytes.  (Courtesy  of  D,  T.  MacDougal  and  Forrest  Shreve,  Desert  Laboratory, 
Carnegie  Institution) , 


THE  PLANT  AND  ITS  ENVIRONMENT 


171 


Living  Organisms. — The  cnvironinontal  factors  which  we 
have  been  discussing  thus  far  are  all  lifeless  ones.  Of  the  utmost 
significance  to  the  plant  are  also  the  living  organisms  with  which 
it  is  surrounded.  Its  relations  to  this  organic  environment 
are  many  and  varied.     First  in  importance  is  the  struggle  for 


Fig.  90. — A  parasite.     The  dodder  (Cu.^ruta)  parasitic  on  an  herbaceous  plant. 
Note  the  absence  of  chlorophyll  and  the  reduction  of  leaves  to  scales. 


survival  which  is  taking  place  continually  between  living  things. 
No  plant  exists  by  itself,  reacting  only  to  the  inorganic  factors 
which  surround  it.  It  is  competing  with  its  neighbors  for  water, 
for  nutrient  materials,  for  sunlight,  for  insect  visitors,  and  for 
other  necessities.  It  is  prej^ed  upon  by  parasitic  plants.  It  is 
devoured  or  destroyed  by  animals  of  all  kinds.  Only  the  vigorous 
and  the  fortunate  succeed,  and  they  are  few  compared  with 
the  hosts  which  fail  and  die. 


172 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Parasites. — The  great  majority  of  the  seed  plants  are  indepen- 
dent, deriving  their  sustenance  directly  from  the  inorganic 
environment,  and  the  struggle  between  them  is  therefore  fair 
combat,  with  victory  to  the  most  efficient.  Some  species, 
however,  have  abandoned  this  passive  form  of  competition  for 
active  attack  upon  their  neighbors  and  have  developed  an  ability 


'   y-/.^^^^         W^BBu 

1^^ 

MmM^bI  fl  V'^^^^wSifHil^Mil 

^B^jSj^ 

gp^^^^r^_^v^sHuH[HK 

l^p^s^^t' 

w  n^ 

y^^B^^ 

i^^^B^^^^^^^HSHI 

^^^^^^ 

^^SB 

I^^P 

**              MMr  ^9^S^^M 

^^^ 

m^  \                        •:, 

1;  *iS 

'<-  if 

Fig.  91. — A  parasite.     The  American  mistletoe  {Fhoradcndron),  parasitic  on 

tree. 


to  obtain  part  or  all  of  their  food  directly  from  the  tissues  of 
other  plants.  Such  an  organism  is  known  as  a  parasite  (Figs. 
90  and  91)  and  its  victim  as  its  host.  The  tissues  of  the  host 
plant  are  pierced  by  small,  modified  roots,  the  sucking  organs  or 
haustoria,  which  may  be  developed  either  from  the  root  or  the 
stem  of  the  parasite.  These  penetrate  to  the  vascular  bundles 
or  storage  regions  of  the  host  and  draw  therefrom  a  supply  of 
manufactured  food.  Parasites  display  certain  characteristic 
structural  modifications,  notably  an  absence  or  poor  development 


THE  PLANT  AND  ITS  ENVIRONMENT 


173 


of  normal  roots,  u  reduction  in  size  of  the  leaves,  and  a  partial  or 
complete  loss  of  chlorophyll.  Some  parasites,  particularly 
those  whose  roots  attack  the  roots  of  other  plants,  may  be  only 
partially  parasitic,  whereas  others  derive  their  entire  food  supply 
from  their  hosts  and  can  live  only  as  parasites. 

The  small  but  interesting  group  of  insectivorous  plants  have 
gone  a  step  further  and  reversed  the  ordinary  relation  between 
animals  and  plants  by  becoming  parasites  upon  insects.  These 
plants  capture  their  prey  either  by  a  closing  trap,  as  in  the 


Fig.  92. — An  insectivorous  plant,  the  sundew  (Drosera).     In  the  sticky  tentacles 
of  its  leaves,  insects  become  entangled. 


Venus's  Fly  Trap;  a  pouch  of  hquid,  as  in  the  Pitcher  Plants,  or  a 
mass  of  sticky  tentacles,  as  in  the  Sundews  (Fig.  92).  Once 
caught,  the  bodies  of  the  insects  are  apparently  digested  by 
enzymes  secreted  by  the  plant,  and  may  thus  furnish  a  small 
supply  of  nitrogenous  food.  Parasitism  is  comparatively  rare 
among  seed  plants  but  is  very  common  in  the  fungi,  many  of 
which  attack  the  tissues  of  both  plant  and  animal  hosts,  pro- 
ducing serious  bacterial  and  fungous  diseases. 

Saprophytes. — Similar  in  certain  respects  to  parasites  are  a 
group  of  plants  which  also  depend  upon  other  organisms  for 
food  instead  of  manufacturing  it  independently  from  raw  mate- 
rials. This  group,  however,  does  not  attack  living  animals  and 
plants  but  lives  instead  upon  their  dead  bodies,  drawing  there- 
from the  already  combined  organic  compounds  and  using  them 
directly  as  food.  Such  plants  are  known  as  saprophytes.  Here 
belong  the  bacteria  of  decay  and  all  bacteria  and  other  fungi 
which  are  not  parasites.  There  are  a  few  saprophytes  among 
seed  plants,  of  which  the  Indian  Pipe  (Fig.  93)  is  perhaps  the 


174 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


best-known  example.  The  structural  modifications  of  such 
forms  are  in  general  similar  to  those  which  distinguish  parasites. 
The  ability  possessed  by  both  parasites  and  saprophytes  to  use 
complex  organic  substances  directly  is  nearly  or  quite  lacking 
among  ordinary  green  plants,  which  are  able  to  take  through 
their  roots  only  simple  inorganic  salts. 


Fig.  93. — Saprophytes.     The  Indian  Pipe  (Monotropa). 


Epiphijtes. — A  third  type  of  relationship  between  one  plant 
and  another,  and  one  which  is  free  from  destructive  consequences, 
is  presented  by  those  species  which  grow  upon  the  bodies  of 
other  plants  but  are  not  parasitic  thereon.  Such  plants  are 
known  as  epiphytes  (Fig.  94)  and  arc  especially  common  in  dense 
tropical  forests.  Many  ferns  and  orchids  display  this  habit  of 
growth.  The  roots  of  epiphytes  have  no  connection  with  the 
ground  and  do  not  enter  the  living  tissues  of  other  plants,  and  in 
consequence  they  are  often  modified  to  draw  moisture  directly 


THE  PLANT  AND  ITS  ENVIRONMENT 


175 


from  the  rain  and  dew.  Structural  features  characteristic  of 
these  plants  are  a  much  thickened  cuticle,  protected  stomata,  and 
fleshy  stems  and  leaves. 

Sijmhiosis. — In  the  relationships  between  the  plant  and  other 
organisms  in  its  environment  which  we  have  mentioned,  the 
advantage  has  been  one-sided.  There  are  instances,  however, 
of  true  symbiosis,  an  intimate  relation  between  two  plants,   or 


Fig.   94. — Epiphytes  growing  on  a  tree-tiiink. 


between  a  plant  and  an  animal,  where  the  advantage,  to  some 
extent  at  least,  is  mutual.  A  notable  example  of  this  is  provideil 
by  the  whole  group  of  Lichens,  each  member  of  which  is  a  com- 
posite organism  produced  by  the  close  association  of  a  species 
of  alga  and  a  species  of  fungus  (Fig.  182)  and  in  which  both 
seem  to  derive  a  certain  amount  of  benefit  from  the  union.  The 
mycorrhiza,  or  association  between  a  fungus  and  the  root  tip  of  a 
higher  plant,  which  we  have  mentioned  in  a  previous  chapter,  is 
evidently  another  example  of  the  same  state,  as  is  probal)ly  the 
connection,  between  the  nitrogen-fixing  root-tubercle  bacteria 
and  the  leguminous  plants  in  which  they  live. 

Passing  higher  in  the  vegetable  kingdom,  wc  find  many  instan- 
ces of  relationship  between  organisms  which  are  also  apparently 


176 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


THE  PLANT  AND  ITS  ENVIRONMENT  177 

of  advantage  to  both  though  they  are  not  intiniati!  enough  to 
class  as  true  syni])ioses.  Most  notable  are  those  between  flower- 
ing plants  and  insects,  whereby  the  plant  derives  the  benefit 
of  cross-pollination  and  the  insect  a  supply  of  nectar;  and  the 
numerous  cases  where  fruits  are  attractive  to  animals  by  furnish- 
ing them  with  food,  and  are  in  turn  distributed  widely  by  these 
animals,  with  consequent  benefit  to  the  plant  species.  These 
two  types  of  relationship  are  very  numerous  and  fascinating  and 
have  long  been  the  subject  of  investigation  by  botanists.  We 
shall  describe  them  more  fully  in  a  later  chapter. 

In  this  brief  discussion  of  the  influence  upon  plants  of  the 
various  factors  in  their  environment,  we  have  merely  outlined 
some  of  the  important  aspects  of  the  problem  which  underlies 
the  whole  science  of  ecology.  No  one  of  these  factors  is  para- 
mount, and  the  reaction  of  the  plant  is  to  the  whole  series  of  them 
(Fig.  95).  The  direction  of  growth,  for  example,  may  be  influ- 
enced both  by  gravity  and  by  light,  and  the  result  is  determined 
by  the  relative  strength  of  these  two  stimuli  under  the  particular 
conditions  existing  in  a  given  case.  In  the  same  way,  the 
distribution  of  a  given  plant  species  over  the  earth's  surface  is 
determined  not  alone  by  soil  conditions  or  moisture  or  tem- 
perature or  parasites,  but  by  the  whole  series  of  environmental 
factors  taken  together.  The  essential  point  to  remember  is 
that  the  structure  and  activities  of  any  plant  are  not  entirely 
controlled  either  by  the  environment  in  which  it  lives  or  by  the 
specific  and  inborn  characteristics  of  its  protoplasm,  but  are 
results  of  an  interaction  between  these  two  forces.  Plants 
belonging  to  different  species  will  develop  and  function  very 
differently  even  under  environments  which  are  identical;  and 
plants  entirely  similar  will  become  very  different  if  grown  under 
different  environments.  We  are  here  facing  the  ancient  problem 
of  Heredity  versus  Environment,  which  confronts  man  in  so  many 
fields  of  incjuiry. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

487.  Give  an  example  of  the  response  of  a  plant  to  its  environment 
which  is  regulatory  in  character. 

488.  What  do  we  mean  by  saying  the  "no  nervous  system  has  been 
differentiated"  in  plants? 

12 


178  BOTANY:  PRINCIPLES  AND  PROBLEMS 

489.  From  its  structure,  whicli  of  the  tissues  of  the  phint  do  you  think 
might  serve  most  readily  as  a  moans  of  transmitting  stiumU  quickly 
from  one  part  of  the  plant  to  anotlier,  in  somewhat  the  same  way  as  do 
the  nerves  among  animals? 

490.  Is  there  anything  in  the  plant  corresiDonding  at  all  to  the  sense 
organs  of  an  animal? 

491.  Give  an  example  of  an  adaptation  of  a  plant  species  to  the 
environment  in  which  it  lives. 

492.  What  danger  may  there  be  for  a  plant  species  in  becoming  very 
closely  adapted  to  a  particular  environment? 

493.  Why  are  plants  more  susceptible  to  frost  when  water  is  abundant 
in  their  tissues  than  when  it  is  not? 

494.  Rank,  rapidly  growing  parts  of  plants  are  apt  to  be  injured  first 
by  frost.     Wliy? 

495.  Why  is  a  slight  frost  in  fall  or  spring  often  more  disastrous  to 
plants  than  very  much  lower  temperatures  in  the  middle  of  the  winter? 

496.  Why  wiH  a  warm  period  in  midwinter  often  prevent  a  fruit  tree 
from  bearing  fruit  the  next  season? 

497.  It  is  better  not  to  fertilize  trees  or  shrubbery  heavilj^  in  the  late 
summer  and  fall.     Why? 

498.  A  frost  while  fruit  trees  are  just  in  bloom  is  more  harmful  than 
it  would  be  a  little  earlier  or  a  little  later.     Why? 

499.  Why  do  the  first  frosts  of  autumn  usually  come  in  low  land? 

500.  Wliy  will  an  orchard  of  such  fruit  trees  as  the  peach,  which  are 
rather  susceptible  to  cold,  survive  better  if  grown  on  a  hilltop  or  on  the 
north  slope  of  a  hill? 

501.  Which  do  you  think  will  withstand  frosts  better,  plants  of  salt 
marshes  or  of  ordinary  soil?     Why? 

502.  Of  what  advantage  and  of  what  disadvantage  is  snow  to 
vegetation? 

503.  Many  of  the  higher  animals  can  thrive  and  function  normally 
in  the  winter  whereas  the  higher  plants  cannot.  To  what  physiological 
difference  between  the  two  groups  is  this  due? 

504.  Of  what  advantage  is  it  to  the  plant  to  have  its  stems  positively 
phototropic  and  its  roots  negatively  so? 


THE  PLANT  AND  ITS  ENVIRONMENT  179 

505.  Since  most  stems  are  positively  phototropic,  why  is  it  that  all 
the  trees  in  the  northern  hemisphere  are  not  bent  somewhat  toward  the 
south? 

506.  Flowers  are  generally  positively  phototropic  and  fruits  negatively 
so.     Of  what  advantage  are  these  reactions  to  the  plant? 

507.  When  nurserymen  grow  seedlings  of  forest  trees,  they  cover  the 
plants  with  a  lattice-work  screen  for  the  first  few  years.     Why? 

508.  Most  plants  in  darkness  grow  abnormallj'  long.  Of  what 
advantage  may  this  reaction  be  to  the  plant? 

509.  Of  what  advantage  is  it  to  the  plant  for  its  roots  to  be  positively 
geotropic  and  its  stems  negatively  so? 

510.  Give  an  example  of  a  stem  which  is  not  negatively  geotropic. 

511.  Give  an  example  of  a  root  which  is  not  positively  geotropic. 

512.  How  different  is  the  direction  in  which  the  trunk  of  a  tree  will 
point  if  it  is  growing  on  a  steep  hillside  from  that  in  which  it  will  i)oint  if 
it  is  growing  on  level  ground?     Explain. 

513.  Just  how  differently  does  the  force  of  gravity  act  when  it  causes 
a  root  to  grow  downward  and  when  it  causes  a  horizontally  placed  piece 
of  wood,  or  other  dead  object,  to  bend  downward? 

514.  Of  what  advantage  to  the  plant  is  it  to  have  its  roots  positively 
hydrotropic? 

515.  Why  do  desert  plants  usually  have  a  ratlier  high  osmotic  con- 
centration in  their  cell-sap? 

516.  Crocus,  narcissus,  and  tulip  plants  flower  and  flourish  in  early 
spring  and  by  late  spring  have  withered  down,  not  to  appear  above 
ground  till  the  following  year.  For  what  sort  of  a  climate  do  you  think 
such  plants  are  well  adapted? 

517.  Why  are  arctic  and  alpine  plants  xerophytic? 

518.  The  leaves  of  many  evergreens,  such  as  juniper,  are  pressed 
against  the  stem  in  winter  and  loosely  spread  in  summer.     Explain. 

519.  The  timberline  (or  line  above  which  trees  do  not  grow  on  a 
mountain  side)  is  generally  higher  on  a  mountain  range  than  on  an 
isolated  peak,  and  in  ravines  than  on  ridges.     Why? 

520.  The  "rosette"  habit  of  growth,  where  all  the  leaves  arc  in  a 
circle  next  to  the  ground  and  a  stem  is  absent,  is  common  in  cokl, 
arid  regions.     ExiMain. 


180  BOTANY:  PRINCIPLES  AND  PROBLEMS 

521.  Why  are  mesophytes  "generally  large,  thrifty  and  fast-growing 
as  compared  with  xerophytes  and  hydrophytes"? 

522.  Which  will  produce  a  better  crop  of  fruit,  a  warm,  dry  summer  or 
a  cool,  moist  one?     Why? 

523.  Why  should  nitrates  and  potash  be  applied  in  large  amounts  only 
early  in  the  growth  of  such  a  crop  as  beans  or  corn,  and  withheld  during 
later  growth? 

524.  What  different  treatment,  as  to  soil  richness  and  moisture, 
should  you  give  to  such  crops  as  celery  and  lettuce  from  that  which  you 
give  to  beans  and  corn?     Why? 

525.  Why  is  a  Plant  Association  often  a  very  heterogeneous  group, 
consisting  of  trees,  herbs,  saprophytes,  climbers  and  many  other  types? 

526.  What  are  the  advantages  and  the  disadvantages  of  a  parasitic 
habit  of  life  to  a  plant? 

527.  In  the  case  of  a  parasitic  plant,  how  are  the  haustoria  able  to 
penetrate  the  body  of  the  host? 

528.  What  do  you  think  are  the  means  by  which  the  parasite  with- 
draws food  and  water  from  its  host  plant? 

529.  Which  type  of  plant  do  you  think  appeared  first  in  evolution, 
the  saprophyte  or  the  parasite?     Why? 

530.  What  important  roles  do  saprophytes  play  in  the  economy 
of  nature? 

531.  Why  are  epiphytes  more  common  in  dense  tropical  forests 
than  anywhere  else? 

532.  Of  what  value  to  a  pine  tree  is  its  pitch? 

533.  Of  what  advantage  to  a  plant  may  be  poisonous  substances 
occurring  in  its  leaves  or  seeds? 

534.  Give  at  least  three  reasons  why  plants  which  are  crowded 
together  do  not  grow  as  well  as  those  which  have  plenty  of  room. 

535.  What  barriers  hinder  the  dispersal  of  marine  plants? 

536.  What  various  reasons  can  you  think  of  to  explain  the  fact  that 
some  plant  species  are  much  more  widely  dispersed  than  others? 

537.  There  are  many  more  species  of  plants  in  the  tropics  than  there 
arejn  temperate  regions.     Explain. 


THE  PLANT  AND  ITS  ENVIRONMENT  181 

REFERENCE  PROBLEMS 

80.  The  mean  annual  temperatures  ("average"  temperature)  of  England 
and  of  New  England  are  not  very  different.  Many  delicate  plants  will  grow 
in  the  former  region  but  not  in  the  latter.     How  do  you  account  for  this? 

81.  Compare  the  United  States  and  the  Britisii  Isles  as  to  rainfall  and 
temperature  during  the  growing  season.  How  do  these  figures  explain  the 
fact  that  grass  grows  so  much  better  there  than  here,  and  corn  so  much 
better  here  than  there? 

82.  What  is  meant,  in  ecology,  by  a  succession  of  plant  associations  and 
what  causes  such  a  succession? 

83.  Although  a  much  wider  range  of  plants  can  Ijc  grown  in  northern 
Europe  than  in  the  northern  United  States,  the  latter  region  has  far  more 
native  species  than  the  former.     Explain. 

84.  As  a  general  rule,  which  can  be  used  in  agriculture  over  a  wider 
area  a  breed  of  livestock  or  a  variety  of  cultivated  plants?     .Wliy? 

85.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Phototropism  Etiolation  Parasite 

Geotropism  Xerophyte  Sapropiiyte 

Hydrotropism  Hydrophyte  Epiphyte 

Chemotropism  Mesophyte  Symbiosis 


CHAPTER  X 
REPRODUCTION 

Thus  far  we  have  studied  the  plant  body  as  an  individual,  in 
which  roots,  stems,  and  leaves  develop  characteristically  and 
function  in  such  a  way  that  the  successful  existence  of  the  plant 
is  assured.  The  individual  ultimately  disappears,  however,  and 
it  is  obviously  necessary  that  if  the  species  to  which  it  belongs  is 
to  survive  and  maintain  itself,  some  means  must  be  provided  for 
insuring  a  constant  succession  of  new  individuals  by  transmitting 
the  life  of  one  generation  to  another.  This  is  accomplished  by  the 
process  which  we  know  as  reproduction,  whereby  the  plant,  by 
one  means  or  another,  produces  a  group  of  offspring.  The 
structures  and  functions  of  all  plants  may  thus  be  divided  into 
two  rather  sharply  marked  groups:  The  vegetative,  centering 
around  the  root,  stem,  and  leaf,  which  are  concerned  primarily 
with  the  life  of  the  individual  plant;  and  the  reproductive,  center- 
ing around  the  flower  and  fruit,  which  are  concerned  primarily 
with  the  maintenance  of  the  species.  It  is  the  latter  group  which 
we  shall  now  discuss. 

There  are  two  main  types  of  reproduction,  markedly  different 
from  one  another.  These  are  asexual  or  vegetative  reproduction, 
in  which  portions  of  the  body  of  the  parent  become  detached  from 
it  and  are  set  apart  as  new  individuals;  and  sexual  reproduction, 
where  there  is  a  union  between  two  speciahzed  reproductive  cells, 
from  which  union  a  new  individual  arises. 

Asexual  Reproduction. — The  simplest  type  of  asexual  repro- 
duction consists  in  the  division  of  the  parent  plant  into  two  or 
more  parts,  each  of  which  becomes  independent.  It  is  a  character- 
istic property  of  most  plants  that  a  small  portion  of  the  body, 
(particularly  if  it  includes  a  bud)  when  removed  and  placed  under 
favorable  conditions,  will  replace  the  missing  parts  and  develop 
into  a  new  individual.  This  readiness  for  multiplication  is  made 
use  of  extensively  in  the  various  arts  of  plant  propagation,  by 
which  new  individuals  are  produced  through  cuttings  and  like 
processes.     In  a  somewhat  similar  manner,  a  bud  or  twig  from 

182 


REPRODUCTION 


183 


one  plant  may  be  united  so  intimately  to  another  by  budding  or 
grafting  that  it  thrives  and  grows  as  an  integral  part  of  the  plant 
to  which  it  has  been  transferred.  By  use  of  these  methods  of 
artificial  reproduction  the  horticulturalist  succeeds  in  producing 
enormous  numbers  of  individuals  from  a  single  plant.  One 
particular  advantage  of  such  a  procedure  is  that  it  insures  a 
high  degree  of  uniformity  among  the  offspring,  since  each  one  of 


Fig.  96. — Asexual  reproduction  in  the  strawberry.  Creeping  stem  (runner  or 
stolon,  s)  from  the  end  of  which  a  new  strawberry  plant  is  growing.  Sexual 
reproduction,  by  flowers  and  fruit,  is  also  shown  on  the  same  plant. 


them  is  in  fact  a  portion  of  the  original  individual.  All  true 
Concord  grape  vines  or  Baldwin  apple  trees,  for  example,  are 
simply  detached  parts  of  the  original  vine  or  tree  in  which  the 
variety  originated. 

In  many  plants,  reproduction  of  this  sort  is  a  constant  accom- 
paniment of  the  slow  spread  and  dispersal  of  the  vegetative  parts 
which  is  continually  taking  place  (Fig.  96).  Thus  a  grass  plant 
may  become  established  in  one  spot  and  gradually  extend  its 
area  until  it  forms  a  considerable  patch  of  turf.  Portions  of 
this,  through  accident  or  decay,  become  separated  from  one 


184  BOTANY:  PRINCIPLES  AND  PROBLEMS 

another,  and  a  colony  of  plants  takes  the  place  of  a  single  indi- 
vidual. In  a  somewhat  similar  fashion  the  beech  tree  multiplies 
itself,  buds  arising  on  the  roots  of  the  parent  tree  imtil  it  is 
surrounded  by  a  grove  of  beeches.  This  method  of  reproduc- 
tion, however,  is  not  very  common  among  the  seed  plants. 

In  many  species  structures  have  arisen  which  are  particularly 
adapted  for  aiding  vegetative  dispersal  of  the  plant  body  and 
which  thus  partake  somewhat  of  the  nature  of  reproductive 
structures.  To  this  group  belong  the  runner  or  stolon  of  plants 
like  the  strawberry,  the  rapidly  spreading  rootstocks  of  the  quack 
grass,  and  the  long,  arching  stems  of  the  blackberry,  in  which  the 
tips  touch  the  ground  and  there  take  root.  Other  vegetative 
parts  are  sometimes  modified  still  further  as  reproductive 
organs.  Perhaps  the  best  known  example  of  such  is  the  tuber 
(Fig.  46)  of  the  potato,  which  is  merely  a  short  and  very  much 
thickened  underground  stem,  from  the  buds  of  which  new  potato 
plants  arise  the  next  season.  The  bulb  (Fig.  47),  as  in  the  onion 
and  hyacinth,  and  the  corm  (Fig.  47),  as  in  the  crocus,  are  also 
short,  stout  stems  with  their  lower  leaves  modified  as  scales. 
They  carry  the  plant  over  from  one  season  to  the  next  and  their 
buds  ultimately  give  rise  to  a  group  of  new  plants. 

Sexual  Reproduction. — Far  commoner  and  more  important 
than  the  asexual  or  vegetative  method  of  reproduction,  however, 
is  the  sexual.  The  essential  feature  here  is  the  union  of  two  special- 
ized sexual  cells,  or  gametes,  to  form  a  single  cell,  the  fertilized  egg 
or  zygote,  from  which  a  new  individual  develops.  To  insure  the 
successful  consummation  of  this  process  is  the  function  of  a  great 
variety  of  reproductive  structures  throughout  the  plant  kingdom. 
These  in  the  higher  plants  we  call  the  flower,  fruit,  and  seed.  We 
are  still  uncertain  as  to  what  notable  advantage  is  gained  through 
this  process  of  sexual  union  which  should  make  it  so  common 
among  plants.  There  is  evidence  in  some  cases  that  an  increase 
in  vigor  characterizes  the  offspring  of  a  sexual  union,  but  just  how 
valuable  this  advantage  is  we  do  not  know,  for  there  exist  many 
very  successful  species  which  now  depend  wholly  or  in  great 
part  upon  asexual  reproduction. 

In  the  present  discussion  we  shall  consider  only  the  higher 
seed  plants  and  shall  reserve  a  study  of  reproduction  in  the 
lower  members  of  the  plant  kingdom  for  later  chapters. 

The  Flower. — Seed  plants  are  characterized  by  the  possession 
of  a  rather  complex  reproductive  apparatus  known  as  the  flower. 


REPRODUCTION 


185 


This  consists  typically  of  those  structures  intimately  concerned 
with  the  development  of  tlu^  sexual  cells,  together  with  others 
which  contribute  indirectly  to  the  success  of  the  process  of  repro- 
duction (Figs.  97  and  98). 

Stamens  and  Pistils. — The  essential  organs  of  the  flower  are  the 
stamens  and  piMiLs.     P^ach  stamen  bears  an  anther  or  pollen  sac. 


|-Anther 
Filament 


-S+igma 
.Style 


Petal 


Stamen 


i 

Pi&ti 


-Chambers 
of  Ovary 

■  Ovule 


Cross  Sectio' 
of  Ovary 


Sepal 

Fig.  97. — The  structure  of  the  flower  of  a  dicotyledonous  .secd-phuit  (diagram- 
matic). .4,  face  view  of  the  flower,  showing  its  calyx  of  five  sepals,  its  corolla 
of  five  petals,  its  ten  stamens,  and  its  pistil.  B,  longitudinal  section,  showing  the 
relations  between  the  parts.  1,  receptacle.  2,  calyx.  3,  corolla.  4,  stamen. 
5,  pistil,  with  ovary  cut  lengthwise. 


and  within  this  sac  are  produced  a  great  number  of  minute, 
single-celled  'pollen  grains  (Fig.  99),  from  the  contents  of  each 
of  which  two  male  gametes  ultimately  develop.  The  anther  is 
commonly  supported  by  a  stalk  or  filament.  The  pistil  consists 
of  a  closed  chamber,  the  ovanj,  at  the  top  of  which  is  the  stigma, 
a  structure  adapted  to  catch  and  hold  the  pollen  grains.  The 
stigma  is  often  supported  by  a  stalk  or  sti/le.  In  the  ovary  are 
borne  the  ovules  or  potential  seeds,  within  each  of  which  is  a 
female  gamete  or  egg.  The  fertilization  of  an  egg  by  a  male  gamete 
starts  the  series  of  processes  which  result  in  the  (l(>v(>l()pin(Mit  of 
the  ovule  into  a  seed. 


186 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Chromosome  Reduction. — Certain  noteworthy  differences  occur 
between  the  cell  division  which  preceeds  the  formation  of  the 
gametes  and   those   which   we    have    studied   in   the    ordinary 


Fig.  98. — The  structure  of  a  flower  of  a  monocotyledonous  seed-plant  {Tril- 
lium). A,  face  view  of  the  flower  showing  calyx  of  three  sepals;  corolla  of  three 
petals;  5  stamens,  and  the  pistil.  B,  side  view  of  the  flower  with  one  petal  and 
one  stamen  removed.  C,  a  transverse  diagram  of  the  flower,  the  sepals  and  ovary 
walls  black,  the  petals,  stamens  and  ovules  outlined.  D,  a  longitudinal  diagram 
of  the  flower.      1,   sepal.      2,  petal.     3,   stamen.       4,  pistil. 


Fig.  99. — Pollen  grains  of  various  types.  A,Circaea.  B,Cobaea.  C,  Morinda. 
D,  Cucurbita.  E,  Pinus.  F,  Dianthus.  G,  Gentiana.  H,  Corydalis.  I, 
Nymphaea.     J,  Taraxacum.     K,  Buphlhalrmmi.     L,  Hibiscus.  ' 


vegetative  tissues  of  the  plant ;  and  a  knowledge  of  these  differ- 
ences is  essential  to  a  thorough  understanding  of  the  laws  of 


REPRODUCTION 


187 


iiilieritaiico,  which  we  shall  later  discuss.  In  this  division  the 
chromosomes,  as  they  make  their  appearance  out  of  the  nuclear 
network,  are  grouped  in  pairs;  and  when  division  takes  place, 
the  members  of  each  pair  separate,  one  going  to  one  of  the 
newly  formed  nuclei  and  the  other  to  the  other.     The  splitting 


^ 


E  F  & 

Fig.  100. — Diagram  of  mitosis  in  a  ordinary  bodj^-cell.  A,  resting  nucleus. 
B  and  C,  prophases.  D,  metaphase.  E  and  F,  telophases.  G,  new  cells.  The 
separate  chromosomes,  each  of  which  has  an  individuality  of  its  own,  are  differ- 
ently marked.  It  is  evident  that  the  chromatic  material  is  divided  exactly 
evenly  between  the  two  daughter  cells.      {Modified  from  Sharp). 

of  chromosomes  which  occurs  in  ordinary  mitosis  (Fig.  100),  and 
by  which  the  chromosome  number  is  maintained,  does  not  occur 
here,  and  the  resulting  daughter  nuclei  (and  the  gametes  de- 
rived from  them)  therefore  contain  only  half  the  chromosome 
number  found  in  the  ordinary  body  cells  of  the  plant.  Such  a 
division  is  accordingly  known  as  a  reduction  division  (Fig.  101). 
When  the  gametes  later  unite  in  fertiUzation,  each  contributes  its 
quota  of  chromosomes,  and  in  the  fertilized  egg  the  original  chro- 
mosome number  is  thus  restored  and  then  persists  throughout 
all  the  cells  of  the  plant  which  develops  therefrom.  The  essential 
differences  between  these  two  types  of  division  are  shown  in  the 
appended  diagrams. 

Perianth. — The  pistil  occupies  the  center  of  the  flower,  with 
the  stamens  in  a  circle  around  it;  and  outside  of  these,  in  turn, 
is  the  'perianth,  composed  typically  of  two  circles  of  parts.  The 
inner  one  of  these  is  the  corolla  or  circle  of  petals.  The  petals  are 
flat,  somewhat  leaf-like  structures,  usually  conspicuous  in  color 
and  rather  delicate  in  texture,  whose  chief  function  is  to  attract 


188 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


to  the  flower  those  insects  which  are  important  in  effecting  polH- 
nation.  Finally,  outside  the  corolla  is  the  calyx  or  circle  of  sepals. 
These  are  usually  green  or  greenish  structures  which  protect  the 
delicate  inner  organs  of  the  flower  while  in  the  bud.     All  floral 


Jl 


.i 

4 

t 

f 

H  I 

P*IG.  101. — Diagram  of  the  "reduction  division",  which  occurs  in  the  develop- 
ment of  reproductive  cells.  Formation  of  a  tetrad  or  group  of  four  pollen  grains. 
A,  resting  nucleus  in  the  pollen  mother-cell.  B,  the  six  chromosomes  become 
distinct,  each  with  an  individuality  of  its  own.  C,  these  chromosomes  group 
themselves  into  three  pairs.  D,  the  three  pairs  become  arranged  across  the 
equator  of  the  cell.  E,  the  members  of  these  pairs  separate,  three  going  to  one 
pole  and  three  to  the  other.  F,  two  new  cells  are  formed,  each  of  which  has  just 
half  the  chromosome  content  of  the  mother-cell,  as  a  result  of  this  reduction 
division.  G,  H,  and  /:  these  cells  each  divide  in  two  normally,  producing  four 
cells  each  with  the  reduced  chromosome  number.  J,  four  pollen  grains  which 
have  thus  arisen  from  the  pollen  mother-cell.     (Modified  from  Sharp). 


parts  are  attached  to  the  receptacle  or  enlarged  tip  of  the  flower 
stalk. 

Variations  in  Floral  Parts. — These  four  groups  of  organs 
exhibit  such  great  differences  in  the  number,  shape,  size,  color, 
texture  and  relative  position  of  their  parts  as  to  give  the  flower  a 


REPRODUCTION 


189 


far  greater  range  of  structural  diversity  than  the  other  organs  of 
the  plant,  and  we  therefore  depend  upon  the  flower  very  largely 
for  those  characters  which  distinguish  genera  and  families  of 
plants  from  one  another. 


Fig.  102. — Diagrams  of  various  types  of  flowers.  The  transverse  diagrams 
show  the  numbers  of  parts  and  the  relations  between  the  members  of  the  same 
circle  of  parts.  The  longitudinal  diagrams  show  the  relations  between  the 
various  circles.  Receptacle  dotted,  petals  and  filaments  outlined,  and  other 
structures  solid  black.  A  and  B,  transverse  and  longitudinal  diagrams  of  the 
flower  of  the  Stonecrop  (Sedum).  Sepals,  petals,  stamens  and  pistils  are  all  free 
from  one  another.  They  are  all  attached  directly  to  the  receptacle,  or  are 
hypogynous.  Each  ovary  is  simple.  C  and  D,  Cherry  (Prunus).  The  sepals 
are  united  into  a  gamosepalous  calyx  but  the 'petals  and  stamens  are'all  separate. 
The  corolla  and  the  stamens  are  attached  directly  to  the  calyx,  or  are  epiacpalous. 
The  ovary  has  but  one  chamber,  in  which  are  two  ovules. 

In  number,  the  circles  may  differ  considerably.  Among  some 
of  the  more  primitive  orders  there  are  several  separate  pistils 
(Fig.  102,  A),  but  this  part  of  the  flower  is  more  often  single, 
at  least  as  far  as  its  ovary  is  concerned,  although  from  the  num- 
ber of  stigmas  or  of  chambers  in  the  ovary  we  have  reason  to 


190 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


believe  that  in  many  cases  there  has  been  a  fusion  of  several 
pistils,  and  that  the  many-chambered  ovary  is  thus  a  compound 
structure  (Fig.  103).  The  stamens  are  often  the  most  numerous 
of  the  floral  parts  and  are  generally  free  from  one  another, 
although  in  some  families  they  may  be  partially  fused  together. 
The  petals  are  usually  fewer  than  the  stamens  and  rarely  exceed 


Fig.  103. — Diagrams  of  various  types  of  flowers.  ^1  and  B,  Blueberry 
(Vaccinium).  The  sepals  are  united  into  a  gamosepalous  calyx,  the  petals  into  a 
gamopetalous  corolla,  and  the  ovary  is  compound,  with  five  cells  or  chambers. 
The  calyx,  corolla  and  stamens  are  united  with  the  ovary,  or  are  epigynous. 
C  and  D,  Tobacco  (Nicotiana).  Calyx  is  gamosepalous,  corolla  gamopetalous 
and  ovary  compound,  with  two  chambers.  Calyx  and  corolla  are  hypogynous 
but  the  stamens  are  attached  to  the  corolla  or  are  epipetalous. 


ten  in  number.  In  certain  orders  they  are  united  to  form  a  con- 
tinuous or  gamopetalous  (as  opposed  to  a  polypetalous)  corolla 
(Fig.  103).  The  sepals  are  generally  of  the  same  number  as  the 
petals  and,  like  them,  may  sometimes  be  fused  together  into  a 
gamosepalous  (as  opposed  to  a  polysepalous)  calyx  (Fig.  102). 
Not  only  are  the  members  of  one  circle  sometimes  joined  together, 


REPRODUCTION 


191 


but  two  entire  circles  may  even  be  united.  The  corolla  is  some- 
times attached  to  the  calyx  and  is  thus  episcpalous  (Fig.  102,  D). 
Similarly,  the  stamens  may  be  epipetalous  (attached  to  the  corolla 
Fig.  103,  D)  and  the  calyx  epigynous  (attached  to  the  ovary,  Fig. 
103,  B)  and  so  on.     In  shape,  floral  parts  vary  enoi'mously.     In 


Fig.  104. — An  irregular  flower,  the  toadflax  [J^uin  m  i  uli/cn  i.s).  Tlic  corolla 
has  two  lips,  which  are  spread  apart  by  the  bee  as  he  enters  in  search  of  the  nectar, 
which  is  secreted  by  a  gland  or  nectary  at  the  end  of  the  long  spur. 


the  higher  plant  groups,  too,  some  of  the  sepals,  petals  or  stamens 
are  different  from  the  rest,  with  the  result  that  an  unsymmetrical 
or  irregular  flower  (Figs.  104  and  240)  is  produced,  as  opposed  to 
the  more  primitive  regular  type  (Figs.  97,  98,  and  109).  In  color, 
flowers  range  through  practically  the  entire  spectrum,  except 
that  green  is  comparatively  rare  in  the  corolla.  In  size  there  is 
also  great  diversity,  although  flowers  more  than  a  decimeter  in 
diameter  are  rare.  In  texture,  flowers  are  generally  soft  except 
for  the  calyx,  the  firmness  of  their  parts  being  produced  by 


192 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


turgidity  of  the  cells  rather  than  by  skeletal  tissues.  In  certain 
cases,  however,  notably  in  the  grasses  and  allied  families,  the 
perianth  segments  have  become  hard,  dry,  and  chaffy. 

Any  one,  or  more  than  one,  of  the  floral  circles  may  sometimes 
be  absent.     If  both  calyx  and  corolla  are  missing  the  flower  is 


Fig.  105. — Cutkias  from  a  male  ijl.int  dI  uiUow  [.Siili.i,  .-Lowing  the  masses 
of  stamens. 

Fig.  106. — Catkins  from  a  female  plant  of  willow.  The  prominent  pistils 
can  readily  be  seen. 


said  to  be  naked.  If  it  is  either  the  stamens  or  the  pistil  which  is 
absent,  the  flower  is  unisexual  and  is  called  either  "male"  or 
"female"  according  to  the  structures  which  it  possesses.  If  both 
male  and  female  flowers  are  distinct  from  one  another  but  are  on 
the  same  plant  (as  in  corn,  birch  and  many  others)  the  plants  are 
said  to  be  monoecious;  if  the  two  sexes  are  on  separate  plants  (as 
in  the  willow.  Figs.  105  and  106)  the  species  is  termed  dioecious. 
In  studying  the  evolution  of  floral  parts,  evidence  has  been 
obtained  that  the  sepals,  petals,  and  probably  the  stamens  and 


REPRODUCTION 


193 


units  of  the  pistil  arc  inoiphologically  leaves;  and  that  the  earliest 
floral  type  was  perfectly  regular,  with  its  various  parts  rather 
numerous  and  with  no  fusion  whatever  between  circles  or  between 
inenil)ers  of  the  same  circle. 

Inflorescence.— T\ie  arrangement  of  flowers  on  the  plant  is 
known  as  the  inflorescence.  The  flowers  may  be  solitary,  arising 
from  the  ground,  or  singly  in  the  axils  of  the  leaves  (Figs.  233 


Fig.  107.— Wind-pollinated  flowers  of  the  alder  {Alnus).  The  long  catkins 
are  groups  of  male  flowers  just  ready  to  shed  their  pollen.  The  smallest  catkins 
are  composed  of  female  flowers,  their  stigmas  ready  to  receive  the  pollen  blown 
through  the  air.  The  woody  cones  of  last  year,  which  developed  from  the  female 
catkins  and  have  shed  their  seed,  are  also  shown. 

and  240);  or  the  leaves  may  be  reduced  to  small  bracts,  the  inter- 
nodes  shortened,  and  the  flowers  thus  grouped  into  definite 
clusters.  Such  clusters  are  of  various  types  as  to  shape  and 
arrangement,  the  commonest  among  them  being  the  raceme 
(Fig.  234),  spike,  head  (Fig.  236),  umbel  (Fig.  235),  corymb, 
panicle,  and  cyme. 

Pollination.— The  first  step  in  the  accomplishment  of  repro- 
duction is  the  transfer  of  pollen  from  the  anthers  to  the  stigma, 
a  process  known  as  pollination.  At  about  the  time  the  flower 
unfolds,  the  anthers  open  and  hberate  the  pollen  grains.  In 
rare  cases  the  stigma  lies  so  close  to  the  anthers  that  the  pollen  is 


194  BOTANY:  PRINCIPLES  AND  PROBLEMS 

transferred  thereto  directly,  and  this  may  sometimes  happen 
even  before  the  flower  opens.  In  the  great  majority  of  eases, 
however,  this  transfer  is  brought  about  by  some  external  agency, 
and  pollen  from  the  flowers  of  one  plant  is  thus  frequently  carried 
to  the  flowers  of  another. 

The  two  most  important  agencies  in  effecting  pollination  are 
the  wind  and  insects.  Wind-pollinated  or  anemophilous  flowers 
(Figs.  107  and  237)  are  prominently  exposed  on  the  plant  but  are 
generally  small,  inconspicuous  and  unisexual,  possessing  a  poorly- 
developed  perianth,  abundant  dry  and  light  pollen,  and  feathery 
stigmas.  Insect-pollinated  or  entomo'philous  flowers  (Figs.  104, 
109,  240,  etc.),  on  the  other  hand,  are  conspicuous  or  possess 
marked  odor,  and  are  characterized  by  a  well-developed  corolla, 
pollen  grains  which  tend  to  adhere  in  masses,  stigmas  which  are 
sticky,  and  in  many  cases  by  the  presence  of  nectaries  secreting  a 
sugary  liquid.  The  insect  is  guided  to  the  flower  by  its  color  or 
odor  and  visits  it  either  to  secure  nectar,  the  source  of  honey,  or 
pollen,  the  source  of  "bee  bread".  Pollen  readily  adheres  to 
the  hairy  bodies  of  these  insects,  and  as  it  is  thus  carried  about 
from  flower  to  flower  it  often  comes  in  contact  with  a  stigma,  to 
the  sticky  surface  of  which  it  is  transferred.  Insects  belonging 
to  the  order  Hymenoptera  (the  bees  and  their  allies)  are  more 
important  than  any  others  in  effecting  pollination. 

In  many  cases  we  have  evidence  that  offspring  which  arise 
from  a  cross,  or  union  of  sexual  cells  contributed  by  two  different 
parents,  are  superior  in  vigor  to  those  in  which  both  gametes 
came  from  the  same  plant.  Perhaps  in  response  to  this  fact, 
there  are  an  enormous  number  of  devices  among  flowering  plants 
which  tend  to  insure  cross-pollination,  or  the  transfer  of  pollen  from 
one  flower  or  plant  to  another,  and  to  prevent  self -pollination,  or 
the  transfer  of  pollen  from  anther  to  stigma  of  the  same  flower. 
Anthers  and  stigmas,  for  example,  may  ripen  at  different  times, 
with  the  result  that  the  anthers  liberate  their  pollen  either  before 
the  stigma  of  the  same  flower  is  ripe  for  pollination  or  after  it 
has  become  no  longer  receptive.  In  many  cases,  also,  pollen 
from  another  plant  is  better  able  to  effect  fertilization  than  the 
plant's  own  pollen;  and  in  extreme  instances  the  plant  may  be 
actually  self-sterile,  its  own  pollen  having  no  effect  whatever 
upon  its  stigma.  More  striking  than  these  methods,  however, 
are  the  multitude  of  structural  devices  in  entomophilous  flowers 
whereby  self-pollination  through  insect  agency  is  rendered  diflS- 


REPRODUCTION 


195 


cult  or  impossible  and  cross-pollination  made  easy.  This  is 
sometimes  accomplished  hy  floral  dimorphism  (Fig.  108),  in  which 
there  are  two  types  of  flowers,  so  constructed  that  the  points 


Fig.  108. — Dimorphic  flowers  of  Chinese  primrose  {Primula  sinensis).  A, 
flower  with  long  style  and  with  stamens  attached  low  in  the  tube  of  the  corolla. 
B,  flower  with  short  style  and  with  stamens  in  the  throat  of  the  corolla. 


Fig.  109. — Flowers  of  the  mountain  laurel  (Kalmia  lalifolia).  The  anthers 
are  held  in  little  pockets  in  the  corolla,  but  the  disturbance  of  the  flower,  as  by 
the  alighting  of  an  insect  upon  it,  will  release  the  stamens  and  they  will  snap 
upward  sharply,  thus  covering  the  insect  with  pollen. 

where  the  anther  and  stigma  touch  the  insect's  body  are  exactly 
reversed,  with  the  result  that  the  pollen  of  one  is  apt  to  reach 
the  stigma  of  the  other.  More  common  are  the  various  and  often 
intricate  devices  in  which  hairs,  springs  (Fig.  109),  traps,  and 
other  agencies  are  employed.     These  reach  their  highest  develop- 


196 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


ment  in  such  families  of  irregular-flowered  plants  as  the  legumes 
and  orchids,  and  have  long  excited  the  curiosity  and  admiration 
of  naturalists. 

Fertilization. — Pollination,  however,  is  only  a  step  toward  the 
union  of  male  and  female  gametes  which  we  know  as  fertilization 


Fig.  llO.^The  process  of  seed-production  in  a  flowering  plant.  Longitudinal 
diagrams  of  flower  and  fruit,  the  calyx  and  corolla  solid  black;  the  ov-ule,  seed- 
coats  and  embryo  dotted,  and  the  ovary  wall,  style  and  stigma  lined.  A,  young 
bud,  the  stamens  and  the  single  ovule  beginning  to  develop.  B,  bud  ready  to 
unfold.  The  embryo-sac  within  the  ovule  is  fully  developed  and  the  egg  (below) 
and  double  endosperm  nucleus  (in  center)  are  ready  for  fertilization.  C,  fully 
opened  flower.  The  anthers  have  burst  and  pollination  has  taken  place,  pollen 
grains  being  transferred  to  the  stigma.  Two  grains  have  germinated,  and  the 
pollen-tube  from  one  of  them  has  penetrated  the  style,  entered  the  ovary,  passed 
through  the  micropyle  of  the  ovule  and  discharged  its  contents — the  two  male 
gametes — into  the  embryo-sac.  Double  fertilization  is  taking  place,  one  male 
gamete  uniting  with  the  egg  and  the  other  with  the  endosperm  nucleus.  D, 
ripe  fruit.  Sepals,  petals  and  stamens  have  dropped  off;  the  ovary  wall  has  hard- 
ened into  the  pericarp;  the  micropyle  has  closed;  the  integuments  have  become 
seed  coats  and  the  ovule  has  developed  into  the  seed.  The  embryo,  in  the  center 
of  the  seed,  has  grown  from  the  fertilized  egg,  and  the  endosperm  surrounding  it 
(shown  in  white)  from  the  endosperm  nucleus. 


(Fig.  110).  Although  the  pollen  grain  is  a  single  cell,  it  is  not  the 
male  gamete.  At  about  the  time  of  pollination,  the  nucleus  of 
the  grain  divides  into  two,  one  of  which,  the  tube-nucleus,  remains 
free  in  the  cytoplasm.     The  other  nucleus  surrounds  itself  with  a 


REPRODUCTION  197 

mass  of  cytoplasm  of  its  own,  sometimes  with  a  separate  wall, 
and  is  known  as  the  generative  cell.  Shortly  after  the  pollen  has 
reached  the  stigma  the  thick  wall  of  the  pollen  grain  bursts  at 
one  point  and  out  of  the  grain  proceeds  a  thin-walled  pollen-tube. 
Near  the  tip  of  this  moves  the  tube-nucleus,  followed  by  the 
generative  cell  (Fig.  111).     This  tube  bores  through  the  tissues 


Fiu.  111. — Germinating  pollen  of  squash.  The  pollen  grain  has  burst  and  a 
pollen  tube  is  starting  down  through  the  style.  Near  the  end  of  the  tube  is  the 
tube  nucleus.  Some  distance  behind  is  the  generative  cell,  from  which  are  later 
developed  the  two  male  cells.      {From  A.  I.  Weinstein). 

of  the  style  and  carries  the  contents  of  the  pollen  grain  down  into 
the  ovary  and  to  the  mouth  of  an  ovule  (Fig.  1 10,  C).  Meanwhile 
the  generative  cell  divides  into  two  male  cells,  which  are  the  true 
male  gametes. 

By  this  time  the  ovule  has  become  prepared  for  fertilization. 
It  possesses  one  or  two  coats  or  integuments,  later  developing 
into  the  seed-coats,  which  cover  it  except  at  one  point,  the  mouth 
or  micropyle.  Here  the  pollen-tube  usually  enters.  Inside  the 
integuments  is  a  thin  nutritive  layer,  the  nucellus.  Within  this, 
in  turn,  and  ordinarily  occupying  the  whole  central  portion  of  the 
ovule,  is  the  embryo-sac.  This  is  a  small,  sap-filled  cavity  with 
three  cells  at  each  end  and  a  naked  nucleus,  the  endosperm 
7uicleus,  near  its  center.  The  three  cells  at  the  end  of  the  sac 
farthest  from  the  micropyle  play  no  part  in  fertilization  or  seed 
development.  Of  the  three  at  the  micropj-lar  end,  however, 
One  is  usually  distinguishable  by  its  greater  size  and  is  the  female 


198  BOTANY:  PRINCIPLES  AND  PROBLEMS 

gamete  or  egg.  With  the  nucleus  of  this  egg  cell  fuses  one  of  the 
male  gametes  which  has  come  down  the  pollen  tube  (Fig.  110,  C). 
This  union  produces  the  fertilized  egg,  and  from  this  single  cell 
develops  the  entire  embryo  of  the  seed  and  thus  the  young  plant 
which  grows  therefrom.  This  fertilized  egg,  in  which  are  com- 
bined the  protoplasm  of  the  two  parents,  is  the  sole  direct  link 
between  parents  and  offspring;  and  only  across  this  exceedingly 
narrow  bridge  are  characteristics  transmitted  by  inheritance 
from  one  generation  to  the  next. 

The  fertilization  of  the  egg  by  a  male  cell  is  not  the  only  cell 
union  which  takes  place  at  this  time,  for  the  other  male  cell 
fuses  with  the  endosperm  nucleus  (Fig.  110,  C),  and  from  the 
cell  thus  formed  arises  the  endosperm  or  food-storage  tissue 
of  the  seed. 

Fertilization  effected  by  gametes  from  the  same  plant  is  known 
as  self-fertilization;  that  by  gametes  from  different  plants  as 
cross-fertilization . 

Seed  Development. — After  fertilization  has  been  effected, 
the  petals  and  stamens  drop  off  and  the  ovule  gradually  develops 
into  the  seed  (Fig.  110,  D).  Various  changes  accompany  this 
process.  The  whole  structure  grows  markedly  in  size  and  the 
integuments  increase  in  thickness,  become  hard  and  woody, 
and  close  over  the  micropyle.  In  many  seeds  a  considerable 
mass  of  endosperm  or  food-storage  tissue  is  developed,  but  in 
others  this  is  much  less  abundant.  Within  the  endosperm  is  the 
embryo  or  young  plant,  which  has  developed  from  the  fertilized 
egg.  In  dicotyledonous  plants  (p.  360)  the  embryo  is  differen- 
tiated into  three  main  portions;  the  hypocotyl  or  primitive  stem 
and  root,  its  tip  directed  toward  the  micropyle;  the  two  seed- 
leaves  or  cotyledons,  attached  to  the  upper  end  of  the  hypo- 
cotyl, and  the  plumule  or  bud,  inserted  between  the  cotyledons 
(Fig.  112).  The  cotyledons  may  be  very  thick  and  serve  entirely 
for  food  storage,  as  in  the  pea;  they  may  be  thin  and  leaf-life, 
serving  as  foliage  leaves  from  the  beginning,  as  in  the  morning 
glory,  or  they  may  combine  both  functions,  as  in  the  squash. 
In  monocotyledonous  plants  (p.  364)  endosperm  is  always  well 
developed  and  the  comparatively  small  embryo  consists  of  a  flat 
disc,  the  scutellum  (which  probably  represents  a  single  cotyledon) 
to  the  face  of  which  are  attached  an  upward-pointing,  sheathed 
plumule  or  bud  and  a  downward-pointing  minature  root  or 
radicle  (Fig.  113).     The  scutellum  serves  to  absorb  food  from 


REPRODUCTION 


199 


the  cndosixM'iii  aiul  to  Iransinit  it  to  the  j>;rowinji;  portions  of  the 
embryo. 

The  ripe  seed  is  thus  a  structure  in  which  the  partially  developed 
young  plant,  well  protected  and  provided  with  an  abundant 
supply  of  food  for  future  growth,  is  able  to  pass  through  a  more  or 
less  extended  period  of  dormancy. 


A  B  C  D  E 

Fig.   112. — Th  estructure  of  a  seed.     A  and  B,  side  and  face  views  of  a  bean 

seed.     C  and  D,  side  and  face  views  of  the  embryo  after  the  seed-coats  have  been 

removed.     E,  the  two  cotyledons  spread  apart,  revealing  the  plumule  within. 

Af ,  Micropyle.     //,  Hilum.     Co^.,  Cotyledons.     T/^/p.,  Hypocotyl.     P/.,  Plumule. 

The  Fruit. — The  ripened  ovary,  together  with  its  contents  the 
seeds,  and  with  any  other  structures  intimately  associated  with 
these,  is  known  as  the  fruit.     The  ripened  wall  of  the  ovary  is 


Fig.  113. — ^A  kernel  (grain  or  fruit)  of  corn.  .1,  face  view,  showing  outline  of 
embryo  in  the  middle.  B,  longitudinal  section.  1,  Pericarp  and  seed  coat,  fused. 
2,  Endosperm.  3,  Plumule.  4,  Scutellum  or  cotyledon.  5,  Radicle.  3,  4  and 
.5  constitute  the  eml)ryo. 


called  the  pericarp.  Fruits  are  various  and  many  different  types 
are  recognized  and  named,  but  we  shall  mention  here  only  the 
most  common  and  important  of  them.  Some  are  dry  at  ma- 
turity and  split  open.  Such  are  the  capsule  (as  in  the  lily), 
which  arises  from  a  compound  ovary,  and  the  pod  (as  in  the 


200 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


bean),  which  arises  from  a  simple  or  single-chambered  one. 
Others  arc  dry  but  do  not  split  open.  Such  are  the  achene  (as 
in  the  buttercup),  the  commonest  type  of  single-seeded  fruit; 
the  nut  (as  in  the  hickory),  in  which  the  pericarp  becomes  hard 
and  woody,  and  the  grain  (as  in  the  corn),  the  characteristic  fruit 
of  the  grass  family,  in  which  seed  coats  and  pericarp  are  firmly 
fused.  These  single-seeded  fruits  are  often  mistaken  for  seeds. 
Many  fruits  become  fleshy,  at  least  in  part.     In  the  bernj  (as 


^^H^  i^^P  iit  ~ 

El 

/:M  '..  •  ',  JM 

^  7^ 

Fig.  114. — Seed  dihpersal  by  the  wind.  A,  npe  fruit  of  the  milkweed,  Ascle- 
pias.  Each  seed  is  provided  with  a  tuft  of  feathery  hairs,  which  aid  in  the  dispersal 
of  the  seeds  by  the  wind.  B,  fruits  of  the  cotton-grass,  Eriophorum.  Each 
tuft  in  the  picture  is  composed  of  a  group  of  single-seeded  fruits,  attached  to  each 
of  which  is  a  cluster  of  long,  cottony  hairs. 

in  the  blueberry),  the  entire  fruit  is  so  except  the  seeds,  which 
have  thick  coats.  In  the  stone  fruit  or  drwpe  (as  in  the  cherry), 
the  outer  part  of  the  pericarp  is  fleshy  but  the  inner  portion, 
enclosing  the  seed,  is  a  hard  and  woody  "stone."  In  the  pome, 
represented  by  such  fruits  as  the  apple  and  pear,  it  is  the  recep- 
tacle, grown  around  and  enclosing  the  fruit,  which  becomes 
fleshy,  the  pericarp  being  represented  here  only  by  the  tough 
membranes  of  the  core. 


REPRODUCTION 


201 


Seed  Dispersal. — It  is  obvious  that  to  leave  successful  offspriuf;- 
a  plant  must  not  only  develop  seeds  but  must  provide  for  their 
dispersal;  and  in  bringing  this  about,  almost  as  great  a  variety  of 
adaptive  devices  are  employed  as  there  are  to  insure  cross- 
pollination.  Dependence  is  placed  upon  various  agencies,  but 
chiefly  the  wind  and  animals.  Many  seeds  or  even  entire  fruits 
are  light  and  provided  with  wings  or  tufts  of  long  hairs,  so  that 
they  present  a  large  surface  for  the  wind  to  catch,  and  are  often 


^^hr         -"''    , 

r^ 

<(;»v 

^i^^^^^yu^^^^E^B 

Fig.  115. — Flower  clusters  of  the 
burdock  (Arctium).  The  bracts  which 
surround  the  cluster  are  stout  and 
hooked,  and  thus  aid  in  the  dispersal  of 
the  fruit. 


Fig. 


110. — Conspicuou.s  berries  of  the 
baneberry  {Aetata) . 


wafted  many  miles  (Fig.  114).  In  the  case  of  the  various 
"tumble  weeds"  the  entire  plant  breaks  off  at  the  base  of  the 
stem  and  is  rolled  along  over  the  ground  by  the  wind.  Other 
fruits  and  seeds  develop  hooks  (Fig.  115),  spines,  or  sticky  secre- 
tions which  enable  them  to  adhere  to  the  fur  of  animals  or  the 
feet  of  birds  and  thus  to  be  carried  for  long  distances.  In  fleshy 
fruits,  the  fleshy  portion  or  pulp  is  usually  bright  in  color  (Fig. 
116)  and  is  rendered  attractive  to  animals  by  its  taste.  Birds 
are  particularly  important  in  the  dissemination  of  the  seeds  of 
such  fruits.  In  a  few  cases  like  the  witch  hazel,  the  fruit  splits 
open  with  such  force  that  the  seeds  are  projected  through  the 
air  for  a  considerable  distance.  The  seeds  and  fruits  of  water  and 
shore  plants  are  usually  dispersed  by  floating  on  the  water  and 
have  been  known  to  travel  thus  for  hundreds  of  miles. 


202 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Seed  Germination. — The  seed  remains  dormant  until  a  favor- 
able environment  appears,  when  the  embryo  begins  to  grow 
and  the  seed  is  said  to  (jerminate  (Fig.  117).  The  conditions 
necessary  for  germination  are  a  plentiful  supply  of  water  and 
oxygen  and  a  moderately  high  temperature.  When  these  are 
fulfilled,  metabolism  begins  vigorously  in  the  embryo  and  in  the 
cells  of  the  endosperm.     Water  is  absorbed  in  large  quantities 


Fig.  117. — Germination  of  the  seed.  A,  the  bean.  1,  embryo  of  seed.  2, 
young  seedling,  the  cotyledons  raised  aljove  the  ground  and  the  plumule  begin- 
ning to  develop.  3,  older  seedling.  .B,  the  pea.  1,  embryo  of  seed.  2,  seedling. 
The  cotyledons  here  remain  in  the  ground,  only  the  plumule  growing  upward. 
{From  Gray) . 


and  the  embryo  swells,  bursts  the  seed  coats,  sends  its  root  into 
the  ground  and  its  stem  into  the  air,  and  becomes  a  seedling. 
The  food  stored  in  endosperm  or  cotyledons  is  digested  and  used 
for  the  development  of  new  organs.  It  is  generally  sufficient  in 
amount  to  provide  for  the  growth  of  the  seedling  to  a  point  where 
the  latter  can  begin  to  manufacture  its  own  food.  Indeed,  as 
soon  as  the  young  stem  and  leaves  get  above  the  ground  they 


REPRODUCTION  203 

turn  green  and  commence  photosynthctic  activity,  soon  supply- 
ing an  abundance  of  food  which  insures  the  rapid  development  of 
the  plant  from  the  seedling  stage  to  maturity,  at  which  point  the 
cycle  of  reproduction  is  complete. 


QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

538.  Is  asexual  reproduction  commoner  in  animals  or  in  plants? 
Explain. 

539.  Give  an  example  of  a  plant  which  commonly  reproduces  itself 
Asexually. 

540.  What  advantage  is  it  to  the  potato  to  reproduce  by  tubers  rather 
than  by  seeds?     What  disadvantage  is  there  in  this  process? 

541.  What  advantages,  aside  from  the  one  mentioned  in  j'our  text, 
may  sexual  reproduction  have  over  asexual  multiplication? 

542.  What  connection  do  you  think  there  has  been  between  the 
stationary  habit  so  characteristic  of  plants  and  their  reproduction  by 
means  of  flowers? 

543.  The  petals  of  a  flower  usuall.v  drop  off  after  seed  is  set  but  the 
sepals  commonly  remain.  Of  what  advantage  are  these  two  facts  to 
the  plant? 

544.  Pollen  grains  are  often  roughened.     Explain. 

545.  Can  you  suggest  what  makes  the  pollen  grain  germinate  and  why 
it  grows  down  the  style  directly  to  the  ovule? 

546.  Why  is  pollen  generally  spoiled  if  it  is  wet  by  the  rain? 

547.  What  effect  does  the  weather  at  apple-blossom  time  have  ui)on 
the  size  of  the  subsequent  apple  crop? 

548.  Do  you  think  that  the  earliest  seed  plants  were  pollinated  by 
wind  or  l)y  insects?     Why? 

549.  What  advantages  and  what  disadvantages  are  there  in  wind- 
pollination? 

550.  Trees  which  are  wind-j^ollinated  usually  flowor  oarly  in  the 
spring.     Explain. 

551.  The  flowers  of  most  coniferous  trees  are  borne  near  the  ends  of 
the  branches,  and  the  flowers  of  grasses  are  usually  raised  up  on  a  tall 
spike.     Explain  these  facts. 


204  BOTANY:  PRINCIPLES  AND  PROBLEMS 

552.  What  advantages  and  what  disadvantages  are  there  in  insect- 
poUination  ? 

553.  The  corollas  of  most  flowers  are  some  other  color  than  green. 
Explain. 

554.  Why  are  low-growing  plants  almost  always  pollinated  by 
insects? 

555.  Alpine  flowers  are  usually  brilliant  in  color  or  otherwise  conspicu- 
ous.    Explain. 

556.  Night-blooming  flowers  are  usually  white.     Explain. 

557.  Low-growing  and  inconspicuous  flowers  are  often  very  fragrant. 
Explain. 

558.  In  many  plants,  the  flowers  are  arranged  in  clusters.  Of  what 
advantage  is  this  to  the  plant? 

559.  In  most  flower  clusters,  the  flowers  open  a  few  at  a  time  rather 
than  all  at  once.     Explain. 

560.  Are  solitary  flowers  usually  larger  or  smaller  than  those  which 
occur  in  clusters?     Explain. 

561.  Do  you  think  that  bees  are  attracted  to  flowers  by  the  same 
odors  which  are  attractive  to  human  beings?  Do  you  think  that  the 
same  holds  true  for  flies?     Explain. 

562.  Most  flowers  do  not  exceed  a  decimeter  in  diameter  and  the  great 
majority  are  very  much  smaller.  Can  you  explain  whj^  flowers  are 
commonly  not  larger  than  this? 

563.  Many  flowers  are  so  constructed  as  to  admit  be'^''  readily  but  to 
exclude  ants.     What  does  the  plant  gain  by  this? 

564.  It  is  a  general  rule  that  plants  rich  in  nectar  tend  to  have  hairy 
stems  and  flower  stalks.     Explain. 

565.  In  many  plants,  the  removal  of  the  stamens  as  soon  as  the  bud 
opens  often  causes  the  flower  to  remain  in  bloom  longer  than  it  would  if 
the  stamens  were  left  attached.     Explain. 

566.  What  means  of  dispersal  have  plants  aside  from  the  dispersal  of 
their  seeds?     Give  examples. 

567.  Other  things  being  equal,  which  type  of  plant  will  become  dis- 
persed more  rapidly,  a  tree  or  an  herb?     Explain. 

568.  What  advantage  is  it  to  a  berry-bearing  plant  to  have  its  fruits 
brightly  colored? 


RErRODrCTION  205 

569.  Wliat  color  i)r('vails  in  uiirii)('  I'ruif  ?     l^xijlaiu. 

570.  Why  is  green  such  an  unconunou  coh^r  among  ripe  berries? 

571.  Fleshy  fruits  usually  have  stony  seeds  or  a  stony  layer  around 
the  seed.     Explain. 

572.  Of  what  use  to  the  plant  is  the  fleshy  food  stored  in  such  a  fruit 
as  that  of  the  apple? 

573.  The  flower  stalks  of  the  dandelion  elongate  after  the  seeds  have 
l)cen  formed.     Of  what  advantage  is  this  to  the  plant? 

574.  As  a  general  rule,  how  tall  are  plants  in  which  the  fruit  bears 
hooks  or  similar  structures? 

575.  Just  what  part  of  the  seed  develops  into  the  new  plant? 

576.  Why  does  cracking  or  chipping  the  shell  of  a  hard-shelled  fruit 
or  seed  often  hasten  its  germination  when  planted? 

REFERENCE  PROBLEMS 

86.  What  relation  has  there  lieen  between  tlie  evolutionary  historj'  of 
insects  and  of  flowers? 

87.  What  flowers  are  there  which  depend  on  flics  rather  than  on  liees 
for  pollination?     How  do  they  differ  from  bee  flowers? 

88.  Does  a  bee  visit  all  kinds  of  flowers,  one  after  the  other,  on  the  same 
day,  or  does  he  confine  himself  to  one  species?  Explain  the  importance  to 
plants  of  this  behavior. 

89.  Some  varieties  of  strawberries  will  set  seeds  when  planted  by  them- 
selves.    Others  will  not.     Explain. 

90.  One  corn  stalk  alone  in  a  field  is  apt  to  produce  few  or  no  seeds. 
Why? 

91.  Cucumbers  and  melons  will  not  fruit  naturally  in  greenhouses,  at 
least  in  winter.     Why  not? 

92.  What  is  Xenia?     Of  what  economic  importance  is  it? 

93.  Give  examples  (other  than  those  in  the  text)  of  a  capsule,  a  ])0(1,  an 
achene,  a  nut,  a  grain,  a  berry,  a  drupe,  and  a  pome. 

94.  Name  three  cultivated  fruits  which  are  csseutially  seedless. 

95.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Calyx  Petal  Hypocotyl 

Corolla  Stamen  Micropyle 

Perianth  Pistil  Cotyledon 

Sepal  Ovule  Plunuile 


CHAPTER  XI 
HEREDITY  AND  VARIATION 

As  a  result  of  the  process  of  reproduction  which  we  have 
described  in  the  preceding  chapter,  a  continuous  succession  of 
new  individuals  arises.  One  of  the  most  remarkable  features  of 
this  reproductive  activity  is  that  each  of  these  new  individuals 
bears  a  very  close  resemblance  to  its  parents.  The  offspring  of 
wheat  plants  are  always  wheat  plants  and  nothing  else;  and  the 
offspring  of  oak  trees  are  always  oaks.  Furthermore,  any 
particular  kind  or  variety  of  wheat  or  of  oak  will  produce  (under 
proper  conditions)  plants  of  that  kind  or  variety.  This  tendency 
for  offspring  to  display  the  particular  characteristics  which 
distinguish  their  parents  is  called  heredity. 

Heredity. — We  have  already  noted  the  exceedingly  narrow 
physical  bridge — the  reproductive  cells  or  gametes — which  con- 
nects one  generation  with  the  next.  To  its  offspring,  one  parent 
contributes  a  single  male  cell  and  the  other  parent  a  single  egg 
cell,  and  out  of  the  fertilized  egg  arising  from  the  fusion  of  these 
two  gametes,  the  new  plant  develops  (Fig.  118).  It  is  evident, 
therefore,  that  the  parental  characteristics  must  in  some  way  be 
transmitted  in  the  protoplasm  of  these  tiny  sexual  cells.  Any 
actual  plant  character  (such  as  redness  of  flower  or  tallness 
of  stem)  obviously  cannot  be  found  in  these  cells;  but  something 
representing  it,  and  capable  of  producing  it  in  the  new  plant, 
must  be  there.  This  "something" — we  are  still  ignorant  of  its 
real  nature — is  called  the  factor  or  gene  for  the  character  in 
question.  In  a  wheat  plant,  let  us  say,  the  height  and  strength 
of  the  stem,  the  shape  and  texture  of  the  leaf,  the  number  of 
spikelets  in  the  head,  the  shape,  color  and  surface  of  the  glumes, 
the  weight  of  the  kernel,  the  character  of  the  grain,  the  yield  of 
seed,  the  resistance  of  the  plant  to  cold,  drought  and  disease, 
together  with  a  host  of  other  characteristics,  have  all  been  shown 
to  be  clearly  inheritable.  It  is  evident  that  in  every  male  gamete 
and  in  every  female  gamete  there  must  be  a  factor  which  repre- 
sents each  of  these  characteristics  and  which  thus  determines  the 
206 


HEREDITY  AND  VARIATION 


207 


Fig.  118. — The  narrow  hereditary  bridge.  The  plant  at  the  right  receives 
from  each  of  its  parents  only  one  minute  sexual  cell,  a  male  gamete  from  one  and 
a  female  gamete  from  the  other.  The  parents,  in  turn,  receive  from  each  of  the 
grandparents  but  one  sexual  cell.  Thus  the  bridge  which  connects  one  generation 
with  the  next,  and  over  which  the  entire  inheritance  must  pass,  is  an  exceedingly 
narrow  one.  The  actual  gametes  are  very  much  smaller,  both  actually  and  in  pro- 
portion to  the  size  of  the  plant,  than  the  dots  by  which  they  are  here  represented. 


208 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


particular  kind  of  wheat  plant  which  is  to  be  produced.  These 
minute  particles  of  protoplasm,  into  which  so  much  is  packed 
and  out  of  which  so  much  emerges,  are  certainly  among  the  most 
remarkable  bits  of  matter  in  existence. 


Fig.    1  !'.•.    -\';iiiati()ii  in  number,  form,  and  size  of  leaflets  in  1  ho  bhie  elderberry, 
Sambucus  glauca.      (From  Bahcock  and  Clausen). 

Variation. — Close  as  the  resemblance  is  between  parent  and 
offspring,  however,  it  is  almost  never  an  exact  resemblance.  Any 
individual  plant  or  animal,  if  studied  closely  enough,  will  be 


HEREDITY  AND  VARIATION 


209 


found  to  differ  somowhat,  oven  thougli  vcM'y  slip;ht.ly,  both  fioin 
its  parents  and  from  its  follow  offspi'ing.  These  differences  are 
known  as  variations  (Figs.  119  and  120)  and  their  presence  brings 
about  that  variability  which  is  so  characteristic  of  all  living  things, 
Laws  of  Inheritance. — The  close  attention  given  to  the  problems 
of  breeding  by  those  who  have  been  responsible  for  the  steady 
improvement  of  our  domesticated  animals  and  plants  through  the 


Fig.  120. — Variation  in  color,  shape,  size,  and  surface  in  the  fruit  of  the 
summer  squash.  All  of  these  types  may  appear  among  the  descendants  of  a 
single  individual. 


centuries  has  been  rich  in  practical  gains,  but  it  has  contributed 
little  to  an  understanding  of  inheritance  beyond  a  recognition  of 
these  two  main  facts  of  heredity  and  variation.  Man  has  long 
known  that  "like  begets  like"  and  that  offspring  differ  among 
themselves,  and  he  has  used  this  knowledge  in  choosing  the  best 
individuals  and  breeding  from  them.  In  this  way  breeders  have 
made  steady  improvement,  but  this  improvement  has  been 
largely  due  to  sharp-sightedness  in  seizing  upon  favorable  varia- 
tions rather  than  to  a  fundamental  understanding  of  inheritance 
which  would  enable  the  breeder  to  control  the  process  and  predict 
its  results.  Within  the  last  century,  however,  and  particularly 
within  the  last  twenty  years,  notable  advances  have  been  made 

14 


210 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


in  our  knowledge,  and  we  are  now  beginning  to  see  that  there  are 
indeed  laws  of  inheritance,  an  understanding  of  which  will  enable 
us  to  raise  the  art  of  breeding  from  clever  but  uncertain  guess- 
work to  such  a  firm  scientific  basis  as  that  upon  which  chemistry 
and  physics  now  rest.  An  investigation  of  these  laws  is  the 
province  of  the  modern  science  of  genetics. 

Inheritance  of  Acquired  Characters.— We  have  learned,  for 
example,  that  all  variations  do  not  behave  alike  in  inheritance. 
Some  are  due  to  factors  embodied  in  the  constitution  of  the 


rf^& 


Fig.  121. — Variation  produced  by  the  environment.  A,  Plant  of  the  dandelion 
(Taraxacum)  grown  in  a  lowland  garden.  B,  Portion  of  the  same  plant  grown  in 
an  alpine  garden,  under  relatively  unfavorable  conditions.     (From  Bonnier). 

gametes  and  may  thus  be  transmitted  from  one  generation  to  the 
next.  They  are  clearly  inheritable  and  are  the  "raw  material" 
with  which  the  breeder  may  work.  Other  variations,  and  among 
them  many  important  ones,  result  from  the  direct  action  of  the 
environment  upon  the  plant  body  during  its  growth  and  are 
apparently  never  transmitted  to  offspring.  Such  "acquired" 
characters  are  the  increased  size  and  vigor  which  result  from 
growth  in  rich  soil  (Fig.  121),  the  thick  and  heavy  leaves  developed 
by  many  plants  when  exposed  to  bright  sunlight,  the  galls  and 
other  malformations  resulting  from  insect  or  fungus  attack,  the 
stunting  effect  of  overcrowding,  and  many  others.     These  varia- 


HEREDITY  AND  VARIATION  211 

tions  merely  affect  the  individual  and  do  not  reappear  in  the 
offspring  unless  the  particular  environment  which  has  caused 
them  persists.  Acquired  variations  of  this  kind  arc  of  particu- 
lar interest  to  the  farmer  or  to  anyone  who  is  concerned  with 
plant  culture,  since  they  can  readily  be  controlled  by  a  proper 
manipulation  of  the  environment ;  but  the  breeder  and  the  student 
of  inheritance  must  learn  to  recognize  them  and  to  realize  that 
they  are  quite  valueless  for  his  purposes.  Good  care  and  cultiva- 
tion will  bring  out  the  best  that  there  is  in  a  poor  race  of  plants 
but  they  can  never  change  a  poor  race  into  a  good  one. 

Mendel's  Law  of  Inheritance.— It  is  therefore  only  with  those 
characters  which  are  clearly  inherited  that  we  are  here  concerned. 
As  we  study  their  behaviour  in  the  passage  from  generation  to 
generation  we  notice  many  apparent  irregularities.  The  same 
parents  will  transmit  different  characters  to  their  different 
offspring.  Sometimes  a  particular  parental  character  will  fail 
to  appear  in  the  offspring  at  all.  In  other  cases  the  offspring 
will  develop  characters  possessed  by  neither  of  its  parents  but 
sometimes  found  in  a  more  remote  ancestor.  It  is  these  facts 
which  geneticists  are  endeavoring  to  explain  and  to  reduce  to 
definite  laws.  The  most  notable  of  these  laws,  and  the  one  which 
is  the  basis  of  our  modern  understanding  of  inheritance,  is  that 
formulated  by  Gregor  Mendel  (Fig.  122),  an  Austrian  monk,  in 
1866.  The  great  importance  of  his  work  was  ignored  at  the  time 
and  was  not  recognized  until  1900.  Since  that  date  a  large 
number  of  investigators  have  diligently  studied  the  applications 
of  "Mendelism"  in  the  inheritance  of  a  wide  variety  of  animals 
and  plants,  and  although  our  advancing  knowledge  has  resulted 
in  many  amplifications  and  interpretations  of  this  law,  it  still 
remains  fundamentally  intact  as  the  cornerstone  of  genetics. 

In  his  cloister  garden  Mendel  studied  inheritance  in  peas, 
making  hj^brids  between  different  types  and  studying  the  results 
from  generation  to  generation.  His  method  of  attack  on  the 
problem  differed  in  several  important  ways  from  that  of  previous 
workers.  First,  in  his  crosses  between  contrasting  types  Mendel 
would  single  out  a  particular  character  of  the  plant  and  follow 
out  its  behaviour  by  itself,  rather  than  trying  to  study  the  whole 
complex  individual  at  once.  In  this  way  the  inheritance  of 
flower  color,  of  seed  color,  of  seed  surface,  of  plant  height,  and  of 
several  other  characteristics  of  the  garden  pea  were  investigated. 
Second,  he  kept  accurate  pedigree  records,  making  sure  that  he 


212  BOTANY:  PRINCIPLES  AND  PROBLEMS 

knew  the  exact  ancestry  of  every  individual  plant  and  the 
characters  displayed  by  each  of  its  ancestors  and  descendants. 
This  method  involves  much  care  and  pains,  both  in  making 
artificially  the  particular  pollinations  desired  and  in  preventing 
all  pollinations  by  such  uncontrolled  agencies  as  insects  and  the 
wind,  and  the  labor  of  keeping  the  records  is  often  very  great. 


Fig.    122. — Gregor  Johann  Mendel,  1822-1884.      (From  Genetics,  by  jicrmission) . 

Mendel's  method  is  now  almost  universally  adopted,  however, 
by  students  of  inheritance.  Third,  in  each  generation  where 
contrasting  characters  appeared  (both  red  flowers  and  white  ones 
in  the  offspring  from  a  single  cross,  let  us  say)  he  carefully  counted 
the  number  of  individuals  of  each  type  and  thus  obtained  a 
mathematical  statement  of  the  facts.  In  short,  Mendel  applied 
the  true  experimental  method  to  the  problems  of  heredity. 

The  results  derived  by  this  novel  and  painstaking  method 
of  investigation  were  carefully  reported  by  Mendel  and  his  inter- 
pretations thereof  have  come  to  be  known  as  Mendel's  Law.  This 
law,  however,  is  not  a  single  proposition  but  really  a  series  of 
distinct  principles.  Its  important  points  we  shall  now  briefly 
discuss. 

Unit  Characters. — ^As  a  general  result  of  his  hybridization 
experiments,  Mendel  observed  that  the  plant  seems  to  behave  in 


HEREDITY  AND  VARIATION  213 

inheritance  as  though  it  were  an  aggregation  of  independent  and 
separable  characteristics,  each  of  which  is  perfectly  distinct  and 
may  exist  with  any  combination  of  other  characters  in  a  given 
individual.  These  traits  he  called  "unit  characters".  We  now 
know  that  the  expression  or  appearance  of  these  characters  may 
vary  considerably  under  different  conditions  and  that  the  real 
unity  lies  rather  in  the  underlying  factor  than  in  the  visible  (and 
perhaps  variable)  character  which  it  produces.  The  essential 
point,  however,  is  that  the  organism,  so  far  as  its  behaviour  in 
inheritance  is  concerned,  seems  to  be  made  up  of  distinct  and 
independent  units.  Purple  flower  color  in  peas,  for  example,  is 
such  a  unit,  and  may  be  associated  with  either  yellow  or  green 
seed  color,  wrinkled  or  smooth  seed  surface,  tallncss  or  dwarfness 
of  vine,  and  so  on.  A  skilful  breeder  may  thus  coml^ine  and 
rearrange  the  characteristics  of  his  plants  almost  at  will. 

Dominance. — Mendel's  studies  also  brought  out  the  fact  that 
when  plants  which  are  dissimilar  in  a  given  feature  (such  as 
flower  color,  let  us  say)  are  crossed,  the  two  characters  thus 
brought  together  differ  markedly  in  ability  to  express  themselves 
in  the  resulting  hybrid  plant.  When  a  pure  purple-flowered 
plant  is  crossed  with  a  pure  white-flowered  one,  for  example, 
all  the  offspring  resemble  the  purple  parent  in  their  flower  color. 
Such  a  character  as  purple  flower  color  in  peas  Mendel  therefore 
termed  dominant  and  one  like  white  flower  color,  which  fails  to 
appear  in  such  hybrid  offspring,  he  called  recessive  (Figs.  123  and 
126).  A  pair  of  contrasting  characters  like  these  are  known  as 
allelomorphs.  All  the  characters  studied  by  Mendel  happened  to 
show  complete  or  almost  complete  dominance  or  recessiveness, 
but  many  instances  have  since  been  found  where  a  hybrid  plant 
resembles  neither  parent  exactly  with  respect  to  a  given  charac- 
ter-pair but  is  more  or  less  intermediate  between  them.  Such 
cases  of  the  incomplete  or  imperfect  dominance  of  one  character 
over  another  in  the  hybrid  state  are  much  more  common  than 
those  in  which  dominance  is  complete.  The  essential  fact  to 
be  emphasized,  however,  and  one  that  is  of  great  practical  import, 
is  that  the  appearance  of  a  plant  (or  animal)  does  not  necessarily 
indicate  its  ancestry  or  its  genetic  make-up.  Dominance, 
partial  or  complete,  may  enable  a  hybrid  or  mongrel  to  masque- 
rade as  a  pure  or  superior  individual. 

Segregation. — Of  much  more  importance  than  this  fact  of 
dominance"  in  the  hybrid  was  Mendel's  discovery  of  the  manner  in 


214  BOTANY:  PRINCIPLES  AND  PROBLEMS 

which  characters  are  transmitted  to  the  second  and  later  genera- 
tions following  a  cross.  The  hybrid  offspring  arising  from  a 
cross  between  a  plant  of  a  purple-flowered  race  and  one  of  a  white- 
flowered  race  are,  as  we  have  said,  all  colored.  In  appearance 
they  resemble  rather  closely  the  purple-flowered  parents,  but  in 
most  such  crosses  the  hybrids  are  somewhat  paler  than  the  pure 
colored  types.  When  two  of  these  hybrid  colored  plants  are 
crossed,  or  when  one  of  them  is  self-fertilized  (which  amounts  to 
the  same  thing  genetically)  both  colored-flowered  and  white- 
flowered  plants  appear  in  their  offspring,  the  former  constituting 
about  three-fourths  and  the  latter  about  one-fourth  of  the  total 
number  of  individuals  in  the  progeny.  These  white-flowered 
plants  breed  perfectly  true  when  self  fertilized,  and  purple  flower 
color  never  appears  in  subsequent  generations  of  their  descendants 
when  inbred.  A  part  of  the  colored  plants  (approximately  one- 
third  of  them)  breed  perfectly  true  to  the  purple  color,  none  of 
their  offspring,  when  inbred,  possessing  white  flowers.  The  rest 
of  the  colored-flowered  plants,  however,  (about  two-thirds  of 
them  and  thus  about  one-half  of  the  total  number  of  the  offspring) 
resemble  the  hybrids  in  color  and  behave  when  self  fertilized 
exactly  as  the  hybrids  did,  producing  offspring  of  which  three- 
fourths  are  colored  and  one-fourth  white.  These  facts  are  set 
forth  diagrammatically  in  Fig.  123. *  This  separation  and  sorting 
out  of  characters  which  occurs  in  offspring  of  hybrid  plants  is 
known  as  segregation.  The  discovery  and  interpretation  of 
segregation  were  perhaps  the  most  important  contributions  which 
Mendel  made  to  our  knowledge  of  inheritance. 

The  essential  character  of  segregation  is  shown  in  the  beha- 
viour of  contrasting  factors  when  they  exist  together  in  a  hybrid 
individual.  A  factor  transmitted  through  the  gametes  of  one 
parent  and  a  contrasting  factor  transmitted  through  the 
gametes  of  the  other  parent,  come  together  and  coexist  in  the 
cells  of  the  hybrid  offspring  plant  without  blending  or  losing  their 
identity;  and  when  such  a  hybrid  plant  produces  its  own  sexual 
cells,  in  turn,  the  two  factors  become  completely  separated  or 
segregated  from  one  another,  each  of  the  new  gametes  containing 
either  the  one  or  the  other  but  never  both.  This  is  well  illustrated 
by  the  example  which  we  have  been  using.  The  factors  for 
purple  and  for  white  flower  color  must  both  be  present  in  the 

*  The  first  generation  following  a  cross  is  technically  known  as  the 
Fi  (first  filial  generation),  the  second  as  the  F2,  the  third  as  the  Fj,  and  so  on. 


HEREDITY  AND  VARIATION 


215 


Purple(PPj  Light  Purple (PW)  Light  Purple(PW) 

if  A^i  Liaht  if  A^i 

Purpl? 

Purple 

Purple 


Fig.  123. — Mendel's  Law  of  Inheritance.  Longitudinal  sections  of  flowers, 
showing  color  of  corolla  and  genetic  constitution  of  the  male  and  female  gametes, 
here  represented  by  ovules  in  the  ovary  and  pollen  grains  in  the  anther.  The 
parents  (Pi  generation)  are  in  one  case  purple-flowered  {FP)  with  the  corolla 
represented  solid  black,  and  in  the  other,  white-flowered  {WW),  with  the  corolla 
merely  outlined.  The  gametes  in  the  former  all  carry  the  factor  for  purple  and 
are  solid  black;  in  the  latter,  all  carry  the  factor  for  white.  In  the  first  hybrid 
generation  or  Fi  (PW),  dominance  of  purple  is  not  complete,  for  the  corolla  is 
colored  but  not  as  darkly  as  in  the  purple  parent.  Its  color  is  represented  by 
cross-hatching.      Note  that  in  the  gametes,  both  male  and  female,  about  half 


216 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fi  hyl)ri(l,  though  only  the  purple  expresses  itself  visibly  in  the 
plant.  Out  of  this  purple  hybrid,  when  self-fertilized,  come  some 
perfectly  pure  white  plants  which  exhibit  no  trace  of  purple  in  their 
descendants  and  some  purples  which  exhibit  no  trace  of  white, 


Parents 


PP 

(Purple) 


WW 

(White) 


Gametes 


All^V 


F, 

F|   Gametes 

PW 
(Purple) 

-^p               iw 

Fz 

(PWxPw) 

ipp 
( Purple) 

ipw 

(Purple) 

(Purple) 

(While) 

Fig.   124. — Diagram  showing  genotype  (in  letters),  and  appearance,  of  parents 
of  the  Fi,  and  of  the  F-i  in  a  cross  between  a  purple-flowered  and  a  white-flowered 


thus  proving  that  the  factors  for  these  characters,  which  for  a 
generation  have  been  existing  together  in  every  cell  of  the  hybrid 
plant,  have  now  (in  one-half  of  the  individuals)  become  com- 
pletely separated  and  have  not  produced  the  slightest  effect  on 
one  another. 


carry  the  factor  for  purple  and  half  for  white,  and  that  none  are  ''light  purple". 
In  the  second  hybrid  generation  or  F2,  derived  by  self-fertilization  of  the  F\, 
about  one  fourth  of  the  plants  are  purple,  with  all  their  gametes  carrying  the 
factor  for  purple;  one  half  light  purple,  with  about  half  their  gametes  carrying 
purple  and  half  white;  and  one  fourth  white,  all  their  gametes  carrying  white. 
The  purple-flowered  plants  breed  true  to  purple  in  the  F3,  the  white-flowered  ones 
to  white,  and  the  light  purple  ones  behave  as  did  the  light  purple  Fi,  producing 
about  one  fourth  purple,  one  half  light  purple,  and  one  fourth  white. 


IIKREDITY  AND   VARIATION  217 

The  denotype. — The  relation  between  this  fact  of  segregation 
and  the  actual  results  which  are  obtained  in  crosses  is  perhaps 
best  explained  if  we  represent  the  factorial  make-up  of  the  plants' 
and  of  their  gametes  by  simple  letters  or  formulas  (Fig.  124) 
in  somewhat  the  same  way  that  Mendel  did  in  his  original  work. 
Every  individual  is  in  a  sense  a  double  structure,  since  it  arises 
from  the  union  of  two  gametes  and  draws  half  of  its  inheritance 
from  one  and  half  from  the  other.  If  we  let  P  represent  the  factor 
for  purple  flower  color  and  W  the  factor  for  white  flower  color,  our 
pure  purple-flowered  parent,  which  received  the  factor  P  from 
both  of  its  parents,  we  might  therefore  represent  by  the  formula 
PP;  and  our  pure  white-flowered  parent  in  the  same  way  by  WW. 
This  formula  applies  to  every  body  cell  of  the  plant.  Of  course 
it  should  be  borne  in  mind  that  we  are  here  representing  only  one 
of  the  great  number  of  factor-pairs  which  are  present  in  the  con- 
stitution of  the  plant.  In  the  cell  divisions  just  preceding  the 
formation  of  the  gametes,  there  is  a  reduction  by  half  in  the 
amount  of  hereditary  material  contained  in  the  nucleus*  and 
every  gamete  produced  now  carries  just /la// of  each  of  the  factor- 
pairs  which  composed  the  parent  plant.  The  gametes  of  the 
purple-flowered  parent  in  our  illustration  would  therefore  all  be 
represented  by  the  formula  P  and  those  of  the  white-flowered  one 
by  W.  When  these  two  plants  are  crossed  and  an  egg,  P,  is 
fertilized  by  a  male  cell,  W,  (or  vice  versa),  the  genetic  formula 
of  the  resulting  hybrid  plant  is  obviously  PW.  Since  purple  is 
almost  completely  dominant  here,  this  plant  appeals  purple- 
flowered,  but  in  its  factorial  make-up  (technically  known  as  its 
genotype)  there  is  a  recessive  factor  for  white.  If  dominance  were 
absent  and  the  hybrid  were  intermediate  in  appearance — pink, 
perhaps — we  should  of  course  still  represent  it  by  exactly  the 
same  genotype.  When  the  two  members  of  a  given  factor-pair  are 
alike,  (as  in  each  of  the  parent  plants  between  which  this  cross 
was  made),  the  individual  is  said  to  be  homozygous  for  the  factor 
in  question;  when  the  two  members  are  different  (as  in  this 
hybrid)  it  is  said  to  be  heterozygous.  Now  the  essence  of  the 
phenomenon  of  segregation  lies  in  the  fact  that  when  this  heter- 
ozygous individual  produces  gametes,  these  are  not  hybrid  or 

*  The  chromosomes  of  the  nucleus  arc  in  all  probability  the  actual  bodies 
in  which  the  genetic  factors  arc  carried,  and  we  have  shown  (p.  187)  that  in 
the  "reduction  division"  just  preceding  the  production  of  gametes,  the 
number  of  chromosomes  in  the  nucleus  is  halved. 


218  BOTANY:  PRINCIPLES  AND  PROBLEMS 

heterozygous  at  all,  but  half  of  them  are  P  and  half  W.  Thus  the 
hybrid  character  of  a  plant  cannot  be  carried  by  its  gametes, 
which  must  be  entirely  one  thing  or  entirely  the  other.  The 
factors  P  and  W,  brought  in  from  the  original  purple  and  white 
parents,  have  coexisted  in  the  hybrid  without  influencing  each 
other  in  the  least  and  have  now  sharply  parted  company,  or 
become  segregated. 

Mendelian  Ratios. — In  a  cross  between  two  of  these  hybrid 
Fi  plants  (or  in  the  case  of  a  self-fertilization  of  one  of  them) 
the  occurrence  of  the  three-to-one  ratio  in  the  F^  generation  is 
thus  easy  to  explain.  Of  the  gametes  of  each  parent,  approxi- 
mately half  carry  the  factor  P  and  half  the  factor  W,  so  that  in  the 
perfectly  free  and  random  union  which  takes  place  between  these 
gametes  there  are  four  possible  combinations  which  may  occur  in 
the  offspring  produced.  P  male  cells  may  fertilize  P  eggs,  pro- 
ducing PP  plants;  P  male  cells  may  fertihze  W  eggs,  producing 
PW  plants;  W  male  cells  may  fertilize  P  eggs,  also  producing  PW 
plants,  or  W  male  cells  may  fertilize  W  eggs,  producing  WW 
plants.  Each  of  these  combinations,  on  the  basis  of  pure  chance, 
is  apt  to  occur  just  as  often  as  any  other.  Approximately  one- 
quarter  of  the  new  generation,  the  PP  plants,  will  not  only  look 
purple  but  will  breed  just  as  truly  for  this  color  as  did  their 
purple-flowered  grandparent;  approximately  one-half,  the  PW 
plants,  will  also  look  purple  (perhaps  somewhat  paler)  but  are  of 
course  heterozygous,  and  when  selfed  or  when  crossed  among 
themselves  will  behave  just  as  did  their  parent,  the  Fi  hybrid, 
and  yield  three  colored-flowered  plants  to  every  white;  and  the 
final  quarter,  comprising  the  WW  plants,  will  appear  white  and 
will  breed  as  truly  to  this  color  as  did  their  white  grandparent 
(Fig.  124).  The  characteristic  Mendelian  ratio  is  therefore  not 
three-to-one  at  all,  but  rather  one-to-two-to-one.  Of  course  it 
should  be  remembered  that  the  results  of  actual  breeding  do 
not  always  display  these  ratios  exactly,  any  more  than  in  the 
tossing  of  coins  or  the  throwing  of  dice  there  are  always  exact 
and  predictable  results.  The  ratios  merely  indicate  what  may 
be  expected  on  the  basis  of  probability. 

Obviously  when  dominance  is  absent  the  F^  generation  will 
not  include  simply  two  sorts  of  plants,  one  three  times  as 
numerous  as  the  other,  but  a  third,  as  well.  A  crimson  snap- 
dragon, for  example,  when  crossed  with  a  white  one  gives  apinkFi 
hybrid.     When  selfed,  this  produces  an  Fo  in  which  one-fourth  of 


HEREDITY  AND  VARIATION  219 

the  plants  are  crimson  (homozygous),  one-half  i)ink  (heterozy- 
gous), and  one-fourth  white  (homozygous),  the  one-to-two-to-one 
ratio  which  we  have  just  mentioned  above.  It  is  evident  that 
pink  is  here  not  a  true  Mendelian  character  at  all,  in  the  sense 
that  it  is  inherited  and  will  segregate,  but  that  it  is  merely  the 
expression  of  two  factors  in  a  heterozygous  condition. 

Independent  Assortment. — When  Mendel  studied  the  inheri- 
tance of  two  or  more  factors  simultaneously  he  discovered  the 
further  important  fact  that  segregation  Avhich  takes  place 
between  the  members  of  any  one  factor-pair  is  quite  independent 
of  that  which  takes  place  in  any  other,  so  that  in  the  second 
generation  from  the  cross  all  sorts  of  recombinations,  many 
of  them  quite  unlike  those  found  in  the  original  parents,  may  occur. 
Let  us  consider  a  plant  which  is  homozygous  for  purple  flowers 
and  also  for  smooth  seeds  and  which  we  may  therefore  represent 
by  the  formula  PP  SS,  to  be  crossed  with  a  plant  homozygous 
for  white  flowers  and  also  for  rough  seeds,  WW  RR  .  The  for- 
mula of  the  Fi  hybrid  offspring  would  of  course  be  PW  SR,  and  as 
smooth  seed  coat  is  dominant  over  rough,  this  plant  would  look 
like  the  purple-flowered,  smooth-seeded  parent.  When  gametes 
are  formed  by  this  plant,  half  of  them  carry  the  factor  P 
and  half  the  factor  W.  But  it  is  clear  that  every  sexual  cell 
must  carry  within  itself  not  only  the  factors  for  flower  color  but 
also  those  for  seed  surface  and  for  all  other  plant  characters, 
as  well,  and  half  of  the  gametes  thus  must  carry  the  factor 
S  and  half  the  factor  R.  Now  we  find  that  in  any  given  sexual 
cell,  it  is  purely  a  matter  of  chance  as  to  whether  the  factor 
for  purple  flowers  is  associated  with  that  for  smooth  seeds  or 
with  that  for  rough  seeds.  The  particular  combination  of 
factors  which  enters  the  Fi  plant  from  each  parent  (purple  with 
smooth  and  white  with  rough,  in  this  case)  has.no  effect  whatever 
upon  the  way  in  which  they  are  associated  in  the  gametes 
produced  by  this  plant.  Their  assortment  is  independent.  Such 
a  plant  as  the  i^i  hybrid  in  this  example  will  therefore  produce 
four  kinds  of  gametes  in  equal  numbers:  P  S;  P  R;  W  S,  and 
W  R.  If  two  such  plants  are  crossed,  there  will  be  sixteen 
possible  combinations  among  their  sexual  cells,  for  there  will 
be  four  kinds  of  pollen  grains  and  four  kinds  of  egg  cells  and 
union  is  quite  at  random.  Any  one  of  these  combinations  is  as 
likely  to  occur  as  any  other,  and  the  sixteen  types  will  thus 
tend  to  be  equally  numerous.     Since  two  of  these  characters  arc 


220 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


dominant,  the  sixteen  types  will  not  all  be  visibly  distinguishable, 
for  the  hybrids  or  heterozygous  plants  will  resemble  the  pure 
dominants.  The  resulting  F2  generation  is  shown  in  Fig.  125. 
Our  expectation  in  such  a  population  is  evidently  that  nine- 


Parents 


PP   5S 

(Purple,  Smooth) 


WW       RR 

(White^Roush) 


Gametes 


PW    SR 
(Purple,  Smooth) 


F;    Game 


F2 

(PWSRx 
PWSR) 


PS 


PR 


tWR 


PS 


iPR 


iws 


ZWR 


r^PPss 

/  Purple,\ 
I  Smooth) 

,iPPSR 
V  Smooth) 

re  PWSS 

(  Purple,  \ 
(smooth) 

re  PWSR 

/  Purple,  N 
(Smooth) 

rePPSR 
/Purple  s 
(Smooth) 

fePPRR 
(  Rough) 

re  PWSR 
(Purple,  X 
ISmooth/ 

riPWRR 
/Purple  \ 
I  Rough  ) 

i  PWSS 

/  Purple,  X 
(Smooth) 

re  PWSR 
VSmoothj 

(White,  N 
(Smooth) 

(Smoom/ 

fePWSR 
(  Purple,  \ 
VSmoothj 

fePlVRR 
/  Purple,  \ 
V  Rough  J 

FsWWSR 
/  White,  \ 
^Smooth) 

li  WW  RR 
/  White,  \ 
I  Rough  j' 

Total  Fz 


15  Purple  Smooth)  j^  Purple,  Rough;  7^  White, Smooth^, 

4  White,  Rough 

Fig.  125. — Diagram  showing  genotype  (in  letters)  and  appearance  of  parents, 
of  Fi,  and  of  F2  in  a  cross  between  a  purple-flowered,  smooth-seeded  pea  and  a 
white-flowered,  rough-seeded  one. 

sixteenths  will  show  both  dominant  characters,  three-sixteenths 
one  dominant  and  one  recessive,  three-sixteenths  the  other  com- 
bination of  dominant  and  recessive,  and  one-sixteenth,  both 
recessive  characters.  The  results  of  another  such  dihybrid 
cross  are  shown  in  Fig.  126.  The  method  by  which  new  combi- 
nations of  characters  are  secured  through  hybridization  is  thus 
clear;  but  we  must  remember  that  many  of  the  F2  plants  are 


HEREDITY  AND  VARIATION 


221 


^ ■^'"^ 


White  Disc 
ww.DD 


Yellow  Sphere 
YYSS 


F,^ 


White  Disc 
WY,  DS 

White  Disc  White  Disc  White  Disc 

WW.DD  WW,DS  WW,DS 


White  Disc 
WY,DD 


White  Disc 
WYDD 


White  Disc 
WY,  DS 


White  Disc 
WXDS 


White  Disc 
WXDS 


Whi+e  Disc 
WYDS 


White  Sphere 
WW5S 


White  Sphere 
WYSS 


2II0W  Disc 

Yellow  Disc 

Yellow  Disc 

Yellow  Sphere 

YY,  DD 

YXDS 

YXDS 

YXSS 

Fig.  126. — Mendel's  law  of  inheritanpe,  where  two  pair.s  of  characters  are 
involved.  In  summer  squashes,  disc  shape  in  fruit  is  dominant  over  sphere 
shape  and  white  color  in  fruit  over  yellow.  A  white  disc  crossed  with  a  yellow 
sphere  gives  an  Fi  generation  all  of  which  appear  white  disc.  Wlien  inbred, 
this  F\  plant  produces  an  F<  generation  which  is  about  ^/fe  white  disc,  "^  le  white 
sphere,  ?f  e  yellow  disc  and  J-j  e  yellow  sphere.  Many  F-;  plants  which  look  alike 
have  very  different  genotypes.  The  appearance  of  the  fruit  and  the  genotype 
of  the  i)lant  from  which  it  came  are  given  in  each  case. 


222  BOTANY:  PRINCIPLES  AND  PROBLEMS 

heterozygous  in  one  or  both  factor  pairs  and  so  will  not  breed 
true  to  their  present  appearance.  The  only  F2  individuals  which 
will  persist  unchanged  when  inbred  are  those  which  are  com- 
pletelj^  homozygous. 


^ 

r- 

Im 

^^^HifT.'^Hk^^^^l 

^m 

1 

'M 

iP 

1 

t;          „      -•               'g^^>^■■ 

^ 

1 

Fig.  127. — A  mutation  in  tobacco.  The  Stewart  Cuban  variety,  which  pro- 
duces an  unusually  large  number  of  leaves  per  plant.  {From  the  Journal  of 
Heredity) . 

Such,  in  brief,  are  the  essential  features  of  "Mendehsm". 
The  intensive  research  of  the  past  twenty  years  in  the  fields  of 
both  botany  and  zoology  has  shown  that  conditions  in  many 


HEREDITY  AND  VARIATION  223 

cases  are  not  as  simple  as  Mendel  found  them  in  garden  peas. 
Some  factors  arc  "linked"  together  and  do  not  display  the  inde- 
pendence of  segregation  which  we  have  noted.  Others  depend 
for  their  expression  not  upon  one  but  upon  a  whole  scries  of 
independent  factors.  Others  are  influenced  in  appearance  and 
inheritance  by  sex.  Size  characters  in  general  (those  of  quantity 
as  opposed  to  quality)  rarely  show  simple  Mendelian  segregation 
at  all  but  blend  more  or  less  completely  and  require  for  their 
investigation  the  use  of  measurements  and  statistical  methods. 


Fig.  128. — "Bud"  sport  or  mutation,  arising  in  a  portion  of  the  plant  and  not 
from  seed.  At  extreme  left  a  loaf  of  the  original  Boston  fern,  and,  at  right,  leaves 
of  three  mutants  which  have  arisen  from  it.     (Courtesy  Brooklyn  Botanic  Garden). 

All  of  these  cases,  however,  can  be  understood  or  explained  by 
amplifying  and  interpreting  Mendel's  original  law  without  at 
all  destroying  its  fundamental  principles,  and  it  remains  toda}^ 
as  one  of  the  most  profound  generalizations  of  biological  science. 
Mutation. — We  have  already  spoken  of  the  variations  produced 
directly  by  differences  in  the  environment  to  which  the  plant 
is  subjected,  variations  which  apparently  are  never  inherited. 
It  is  now  clear  from  our  consideration  of  Mendelism  that  another 
and  doubtless  a  very  common  cause  of  variation  is  the  recom- 
bination of  characters  which  follows  the  crossing  of  two  different 
types  or  races;  and  these  variations,  being  due  to  differences  in 
the  inherent  genetic  factors  themselves,  are  clearly  inheritable 


224 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


HEREDITY  AND  VARIATION  225 

accordiiis  to  (lofinit(>  laws.  ThcM-o  is  still  a  third  type  of  variation, 
often  of  importan(!e  in  nature  and  in  j)ractical  breeding,  which 
we  know  as  mutation.  Frequently  an  individual  clearly  different 
from  the  rest  will  appear  in  a  pure  race,  where  there  is  apparently 
nothing  in  the  ancestry  which  can  explain  its  origin,  and  will 
breed  true  to  this  new  type  in  succeeding  generations  (Fig.  127). 
The  production  of  such  a  new  and  distinct  type  we  call  a  muta- 
tion. Many  "double-flowered  "  races  of  plants  have  arisen  in  this 
way,  as  have  forms  with  cut  leaves  and  a  great  variety  of 
other  characters.  When  the  history  of  such  a  plant  type  can  be 
traced,  it  is  often  found  to  begin  with  a  single  individual  which 
arose  by  mutation  from  the  normal  race  and  has  transmitted 
its  characters  to  its  descendants.  In  some  cases  a  mutating 
individual  is  strikingly  different  from  the  normal  form  and  is 
then  often  called  a  "sport".  In  others,  the  difference  is  so 
small  that  it  can  hardly  be  recognized.  Many  instances  have 
also  been  found  where  the  mutation  appears  in  a  single  branch 
or  portion  of  the  plant  rather  than  in  a  whole  individual  grow- 
ing from  seed  (Fig.  128).  All  mutations  agree,  however,  in 
coming  without  warning  or  evident  cause  and  in  being  trans- 
mitted to  offspring.  By  mutation  have  arisen  some  of  our 
important  horticultural  and  crop  plants  (Fig.  129)  such  as  the 
kohl-rabi,  the  navel  orange,  the  thornless  cactus,  the  moss  rose, 
the  Shirley  poppy,  and  others.  We  can  understand  and  man- 
ipulate, to  a  certain  extent,  the  variations  due  to  environment 
and  to  hybridization,  but  mutations  are  as  yet  beyond  our  con- 
trol. The  best  that  the  plant  breeder  can  do  is  to  watch  for 
them  closely  and  seize  upon  them  when  they  appear. 

The  science  of  genetics  is  today  one  of  the  most  intensively 
studied  branches  of  biology  and  has  not  only  yielded  us  valual^le 
information  as  to  the  laws  by  which  various  characteristics  are 
transmitted  from  parent  to  offspring,  but  through  its  identifica- 
tion of  the  chromosomes  of  the  nucleus  as  the  probable  seat  of 
genetic  factors,  it  has  even  thrown  light  on  the  structure  and 
behaviour  of  protoplasm  itself. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

577.  What  is  the  chief  practical  importance  of  discovering  laws  of 
inheritance? 

578.  In  studies  of  inheritance  in  the  summer  squash  it  has  been 
found  that  white  fruit  color  is  dominant  over  yellow,  and  that  the  differ- 

15 


226  BOTANY:  PRINCIPLES  AND  PROBLEMS 

ence  between  these  two  colors  is  due  to  a  single  factor.  If  a  plant 
homozygous  for  white  is  crossed  with  a  plant  homozygous  for  yellow 
fruit  color,  what  will  be  the  appearance  of  the  Fi  generation?  of  the 
F2  generation  derived  by  self  fertiUzing  one  of  these  Fi  ijlants?  What 
proportion  of  the  white-fruited  F2  plants  will  breed  true  if  self-fertilized? 

579.  What  will  be  the  fruit  color  of  the  plants  produced  by  crossing 
one  of  the  Fi  individuals,  mentioned  in  Question  578,  with  its  yellow- 
fruited  parent?     with  its  white-fruited  parent? 

580.  Let  the  factor  for  white  fruit  color  in  the  squash  be  represented 
by  W  and  that  for  yellow  by  Y.  What  kind  of  gametes,  as  far  as 
their  factors  for  fruit  color  are  concerned,  will  be  produced  by  plants 
having  the  following  genotypes:  WW,  WY,  YY? 

581.  What  gametes  will  be  produced  by  the  plants  involved  in 
the  four  following  crosses;  and  what  will  be  the  fruit  color  of  the  offspring 
from  each  cross: 

WY  X  YY;  WW  X  WY;  YY  X  WW;  WY  X  WY? 

582.  A  white-fruited  squash  plant  when  crossed  with  a  yellow- 
fruited  one  produces  offspring  about  half  of  which  are  white  and  half 
yellow  in  fruit  color.     What  are  the  genotypes  of  the  parent  plants? 

583.  If  the  white-fruited  parent  plant  in  the  preceding  question  is 
self-fertilized,  what  will  be  the  fruit  color  of  its  offspring? 

584.  If  this  same  white-fruited  parent  is  crossed  with  one  of  its  white- 
fruited  offspring  mentioned  in  Question  582,  what  chance  is  there  of 
obtaining  from  this  cross  a  yellow-fruited  plant? 

585.  Two  white-fruited  squash  plants  when  crossed  produce  about 
three-fourths  white  and  one-fourth  yellow  offspring.  What  are  the 
genotypes  of  these  two  parents,  as  to  fruit  color?  What  will  each  pro- 
duce if  crossed  with  a  yellow-fruited  plant? 

586.  A  cross  between  a  white-fruited  and  a  yellow-fruited  squash 
plant  produces  offspring  all  of  which  have  white  fruits.  If  two  of  these 
F\  plants  are  crossed  together,  what  will  be  the  fruit  color  of  Uidr 
offspring? 

'Note. — In  Four-o'clock  flowers,  red  flower  color  is  incompletely  domi- 
nant over  white,  the  hybrids  being  pink-flowered. 

587.  If  a  red-flowered  Four-o'clock  plant  is  crossed  with  a  white- 
flowered  one,  what  will  be  the  flower  color  of  the  Fi?  of  the  F2?  of 
the  Fi  crossed  back  on  the  red-flowered  parent?  of  the  Fi  crossed  back 
on  the  white-flowered  parent? 


HEREDITY  AND  VARIATION  227 

588.  In  Four-o'clock  flowers,  let  R  represent  the  factor  for  red  flower 
color  and  W  the  factor  for  white.  What  will  be  the  flower  color  of  the 
offspring  from  the  following  four  crosses,  in  which  the  parents'  genotypes 
are  given: 

RW  X  RR;  WW  X  RW;  RR  X  WW;  RW  X  RW? 

589.  If  you  wanted  to  produce  Four-o'clock  seed  all  of  which  would 
yield  pink-flowered  plants  when  sown,  how  would  you  do  it? 

590.  In  what  respect  is  a  character  which  behaves  like  flower  color  in 
Four-o'clocks  easier  to  deal  with  in  breeding  work  than  one  which  behaves 
hke  fruit  color  in  squashes? 

Note. — In  the  inheritance  of  fruit  shape  in  summer  squash  it  has  been 
found  that  the  "disc"  type  is  dominant  over  the  "sphere"  type  (see 
Fig.  126). 

591.  In  a  cross  between  a  squash  plant  homozygous  for  yellow  fruit 
color  and  disc  fruit  shape  and  a  plant  homozj^gous  for  white  fruit  color 
and  sphere  fruit  shape,  what  will  be  the  color  and  shape  of  the  fruit  in 
the  Fi?  What  will  these  be  in  the  F2?  produced  by  crossing  two  of 
these  Fi  plants  together? 

592.  If  one  of  the  Fi  plants  in  the  preceding  question  is  crossed  back 
onto  its  yellow  disc  parent,  what  will  be  the  color  and  shape  of  fruit  in 
their  offspring?  What  will  these  be  if  the  Fi  plant  is  crossed  back  onto 
its  white  sphere  parent? 

593.  Let  D  represent  the  factor  for  disc  fruit-shape  and  S  the  factor 
for  sphere.  What  will  be  the  color  and  shape  of  fruit  in  offspring  of  the 
following  crosses: 

WW  SS  X  YY  DD  WY  DS  X  WY  SS 

WY  DD  X  YY  SS  WY  DS  X  YY  SS 

WY  DS  X  WY  SS  WY  DS  X  WY  DS 

Note. — In  the  following  six  questions,  all  of  which  deal  with  fruit  color 
and  shape  in  the  summer  squash,  the  appearance  of  parents  and  offspring 
is  stated.     Determine  in  each  case  the  genotypes  of  the  parents. 

594.  White  disc  crossed  with  yellow  sphere  gives  one-half  white  disc 
and  one-half  white  sphere. 

595.  White  sphere  crossed  with  white  sphere  gives  three-fourths 
white  sphere  and  one-fourth  yellow  sphere. 

596.  White  disc  crossed  with  yellow  sphere  gives  one-fourth  white 
disc,  one-fourth  white  sphere,  one-fourth  yellow  disc,  and  onc-fourtli 
yellow  sphere. 


228  BOTANY:  PRINCIPLES  AND  PROBLEMS 

597.  White  disc  crossed  with  white  sphere  gives  three-eighths  white 
disc,  three-eighths  white  sphere,  one-eighth  yellow  disc  and  one-eighth 
yellow  sphere. 

598.  Yellow  disc  crossed  with  white  sphere  gives  all  white  discs. 

599.  White  disc  crossed  with  white  disc  gives  28  white  disc  plants,  9 
white  sphere  plants,  10  yellow  disc  plants,  and  3  yellow  sphere  plants. 

600.  A  cross  between  a  plant  with  white  disc  fruits  and  one  with 
yellow  sphere  fruits  gives  25  plants  with  white  disc  fruits,  26  with  white 
sphere,  24  with  yellow  disc,  and  25  with  yellow  sphere.  If  the  white  disc 
parent  is  self-fertilized,  what  proportion  of  its  offspring  will  have 
yellow  sphere  fruits? 

601.  Explain  how  it  can  be  that  plants  which  look  exactly  alike  may 
breed  very  differently. 

602.  Hybrid  animals  and  plants  notoriously  fail  to  breed  true.     Why? 

603.  If  a  potato  breeder  desires  to  obtain  a  new  variety  of  potatoes 
by  selection  from  among  a  large  number  of  plants,  would  you  advise 
him  to  plant  potato  "seed"  (pieces  of  the  tuber)  or  real  seed  from  the 
seed  capsule,  to  provide  plants  from  which  be  may  select?     Why? 

604.  Do  you  think  that  the  characteristics  of  the  fruits  of  an  apple 
tree  will  be  affected  by  the  kind  of  pollen  which  fertilized  the  flowers? 
Explain. 

REFERENCE  PROBLEMS 

96.  Summarize  briefly  the  hfe  and  work  of  Mendel  and  tell  how  his 
discoveries  were  finally  brought  to  the  attention  of  the  world. 

97.  Give  examples  (aside  from  those  mentioned  in  the  text)  of  new  plant 
varieties  which  have  arisen  as  a  result  of  hybridization;  as  a  result  of 
mutation. 

98.  State  briefly  why  it  is  that  the  chromosomes,  rather  than  any  other 
part  of  the  gametes,  are  believed  to  carry  the  hereditary  factors. 

99.  What  is  a  "Pure  Line"  of  plants?  Of  what  importance  are  Pure 
Lines  in  agricultural  practice? 

100.  What  is  meant  by  "bud  selection"  in  horticultural  practice? 

101.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Heredity  Segregation  Mutation 

Allelomorph  Homozygous  Genotype 


CHAPTER   XII 
EVOLUTION 

Among  all  organisms  which  we  can  carefully  watch  and  study 
new  variations  are  continually  appearing  and  being  inherited" 
This  fact  at  once  suggests  that  living  things  are  not  constant 
and  changeless  in  their  characteristics  but  that  they  may  undergo 
a  certain  degree  of  permanent  modification  as  the  generations 
succeed  each  other.  A  glimpse  at  the  remarkable  development 
of  our  domestic  plants  and  animals,  since  man  first  began  to 
utilize  them  and  to  improve  them  by  selection,  shows  the  possi- 
bilities of  change  which  exist  among  organisms.  The  difference 
between  the  small,  sour  prototype  of  the  apple,  for  instance,  and 
our  modern  large  and  delicious  varieties  is  so  great  that  we  can 
hardly  recognize  the  relationship  between  the  two.  In  fact, 
many  of  our  cultivated  forms  have  progressed  so  far  under  human 
guidance  that  we  do  not  know  what  their  wild  ancestors  were. 

Since  the  days  of  the  Greeks,  philosophers  have  often  specu- 
lated as  to  the  possibility  that  the  whole  organic  world — plants, 
animals  and  man — has  reached  its  present  state  through  a  gradual 
evolution  from  far  simpler  forms,  perhaps  ultimately  from  organic 
matter.  It  is  only  within  the  last  century,  however,  that  the 
subject  of  evolution  has  descended  from  these  clouds  of  specula- 
tion and  become  a  problem  for  scientific  study.  Little  by  little, 
evidence  has  been  accumulating  that  progressive  change  has 
actually  taken  place,  and  that  the  plants  and  animals  which  we 
now  know  differ  radically  from  their  ancestors  in  past  ages.  As 
to  just  how  this  has  been  accomplished  biologists  are  still  far 
from  agreed,  but  as  to  the  fact  of  evolution  there  is  now  practi- 
cally no  doubt  in  the  minds  of  scientific  men.  An  insistence  that 
every  species  was  specially  and  suddenly  created  in  exactly  the 
form  which  it  now  displays  has  given  way  to  the  more  profound 
conception  of  the  world  as  the  theatre  of  a  slow  but  steady  upward 
progress  from  lower  to  higher  types  of  life. 

Evidences  for  Evolution. — The  lines  of  evidence  on  whicli  the 
belief  in  evolution  is  based  are  various,  and  we  shall  briefiy 
discuss  a  few  of  them. 

229 


230  BOTANY:  PRINCIPLES  AND  PROBLEMS 

Geological  Evidence. — Probably  of  most  importance  is  the 
existence  of  fossils,  the  actual  remains  or  impressions  left  in 
the  rocks  by  ancient  plants  and  animals,  caught  and  embedded  in 
the  sand  or  mud  millions  of  years  ago.  As  our  knowledge  of 
geology  becomes  greater,  we  find  that  fossils  do  not  occur  indis- 
criminately but  that  similar  types  appear  in  rock  layers  which  we 
know,  from  their  position,  to  be  of  about  the  same  age.     We 


Fig.  130. — A,  leaf  of  a  fossil  species  of  the  Judas  Tree  (Cercis),  from  the  Eocene 
of  Tennessee.  B,  leaf  of  a  living  species,  Cercis  canadensis,  now  growing  through- 
out the  eastern  United  States.  The  two  species  are  similar  to  each  other  but  are 
clearly  distinct.  Our  living  species  has  probably  been  evolved  from  an  ancestor 
much  like  the  fossil  one  here  shown.     {After  Berry). 

are  able  to  assign  each  rock  layer,  or  stratum,  to  its  particular 
level  in  the  great  series  which  records  geological  history  from  a 
very  remote  past  to  the  present,  and  we  find  as  we  pass  upward 
through  this  series,  from  the  most  ancient  rocks  to  the  most 
recent  ones,  that  the  fossil  remains  change  progressively  as  we 
proceed;  and  that  as  we  approach  modern  times,  the  proto- 
types of  our  familiar  plants  and  animals  begin  to  come  into  view 
(Fig.  130)  until  in  recent  deposits  we  find  as  fossils  species  which 
still  exist.  There  are  enormous  gaps  in  this  record  but  the 
advance  of  geological  science  is  slowly  filling  them  in,  and  even 
now  we  can  catch  a  glimpse  of  the  main  scenes  in  the  pageant  of 
evolutionary  progress.  Among  the  members  of  the  plant  king- 
dom, we  can  witness  the  rise,  luxuriance,  and  extinction  of  several 
great  groups;  we  can  trace  the  development  of  seed  plants  from 
lowly,  fern-like  forms,  and  we  can  recognize  approximately  the 


EVOLUTION  231 

point  at  which  our  modern  flowering  plants  fii'st  appeared  upon 
the  earth.  No  other  evidence  for  evolution  is  quite  so  convincing 
as  are  these  tangible  remains  of  extinct  organisms. 

Taxonomic  Evide7ice. — The  general  character  and  classification 
of  the  plant  and  animal  kingdoms  also  bears  testimony  that 
their  present  state  is  the  result  of  descent,  with  progressive  modifi- 
cation, from  earlier  types.  A  study  of  the  external  and  internal 
structure  of  living  things  makes  it  clear  that  they  are  not  hap- 
hazard and  random  in  their  characteristics,  but  that  they  fall 
into  well-marked  groups  of  similar  forms,  the  members  of  which 
show  definite  resemblances  to  each  other.  All  similar  indivi- 
duals we  class  together  as  a  species.  A  number  of  species  resem- 
ble one  another  so  much,  and  are  so  different  from  anything  else, 
that  we  place  them  together  as  a  genus.  A  number  of  genera, 
in  the  same  way,  stand  apart  as  a  family.  Families  are  united 
into  orders,  orders  into  classes,  and  so  on.  We  can  understand 
this  grouping  of  similar  species  and  their  union  into  progressively 
larger  aggregations  if  we  regard  the  organic  world  as  a  huge 
"genealogical  tree",  its  members  related  to  one  another — some 
nearly,  some  remotely — by  ties  of  descent,  the  ''twigs"  repre- 
senting species,  which  unite  into  larger  and  larger  branches  as  we 
trace  them  back  to  the  main  trunk.  These  facts,  which  make  the 
science  of  taxonomy  possible,  are  unexplainable  otherwise. 

Morphological  Evidence. — Equally  significant  are  certain  facts 
which  morphology  presents.  Many  organs  exist  today  in  a 
state  evidently  useless  to  the  plant  or  animal  possessing  them 
and  for  which  it  is  hard  to  account  unless  we  look  upon  them  as 
vestiges  or  remnants  of  structures  which  once  had  a  use  but  have 
lost  it  during  the  course  of  evolution.  Vestigial  stamens,  petals, 
sepals,  stipules,  and  leaf  blades,  as  well  as  various  functionless 
internal  structures,  are  of  frequent  occurrence  in  plants,  and  there 
are  many  similar  instances  in  the  animal  kingdom.  Their 
presence  can  be  explained  only  if  we  assume  that  they  once  were 
well  developed  and  functional  but  that  evolutionary^  progress, 
which  makes  them  necessary  no  longer,  has  resulted  in  their 
gradual  degeneration. 

Evidence  from  Geographical  Distrihuiion. — Impressive  evidence 
in  favor  of  evolution  is  presented  by  the  facts  of  geographical 
distribution.  Most  plant  species  are  not  widely  dispersed  over 
the  earth's  surface,  or  even  over  that  part  of  it  in  which  condi- 
tions  are  well  suited  for  their  growth.     The  golden-rods,  for 


232 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


example,  are  practically  confined  to  North  America,  the  eucalypts 
to  Australia,  the  tobaccos  to  the  western  hemisphere,  and  so  on. 
We  can  explain  such  localized  distribution  only  by  assuming  that 
these  plants  were  evolved  in  the  regions  which  they  now  inhabit, 
and  have  been  confined  there  ever  since  by  barriers  of  various 
sorts  (Figs.   131,   132,  and   133).     The  same  phenomena  occur 


Fig.  1.31. — The  distribution  of  four  closely  related  species  l)elonging  to  the 
same  genus  (Sabatia,  section  Pleienta).  1,  Sahatia  decandra.  2,  .S'.  foliosa. 
3,  ,S'.  dodecandra.  4,  S.  Kennedyana.  These  four  very  similar  but  readily 
distinguishable  species  have  presumably  all  evolved  from  a  common  ancestor, 
which  once  grew  on  the  coastal  plain  of  southeastern  North  America.  {Data  from 
M.  L.  Fernald) . 


repeatedly  throughout  the  animal  kingdom,  and  it  is  certain  that 
the  great  mass  of  facts  which  we  now  possess  on  the  geographical 
distribution  of  organisms  would  be  largely  unexplainable  if  we 
did  not  believe  that  each  species,  genus,  and  family  of  plants 
and  animals  has  had  its  place  of  origin  and  its  own  individual 
evolutionary  history.  The  facts  of  distribution  are  meaningless 
on  any  other  hypothesis. 


EVOLUTION 


233 


Because  of  such  facts  as  these,  the  scientific  world  has  l)ecouie 
convinced  tliai  evolutionary  change  has  actually  occurred  and 
that  the  plants  and  animals  with  which  we  are  now  familiar  an; 
the  most  recent  members  of  innumerable  lines  of  descent,  reaching 


Fig.  132. — Three  of  the  four  spooies  of  Sabatia  the  distribution  of  which  is 
mapped  in  Fig.  131.  They  are  similar  but  quite  distinct  from  one  another. 
1-3,  Sabatia  Kennedyana;  4  and  5,  S.  dodecandra;  6-8,  S.  dccandra.  (From 
M.  L.  Fernaid).  ■ 


backward  for  millions  of  years  and  embracing  a  multitude  of 

ancestral   forms   entirely   different   from    anything   now    alive. 

An  admission  of  the  fact  of  evolution  at  once  raises  two  grave 

(}uestions,    however;   whence   came   the   first   living  thing,   the 


234 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


original  ancestor  of  all  which  have  since  evolved;  and  what  has 
been  the  cause  of  this  steady  and  long-continued  evolutionary 
progress?  The  first  question  involves  the  origin  of  life,  about 
which  we  must  frankly  admit  that  our  ignorance  is  still  complete. 


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Fig.  133.— The  distribution  of  Gaylussacia  dumosa  (lined)  and  its  variety 
Bigeloviana  (dotted).  The  species  is  characteristically  southern,  the  variety 
northern.  In  New  Jersey  and  adjacent  Pennsylvania,  where  their  ranges  over- 
lap, the  variety  merges  into  the  species,  but  in  the  rest  of  its  range  it  is  quite 
distinct.  Here  we  evidently  have  two  groups  of  plants,  within  a  single  species, 
which  occupy  different  regions  and  which  have  diverged  from  one  another  almost 
far  enough  to  be  regarded  as  two  distinct  species.     (,Data  from  M.  L.Fernald). 

The  second,  which  relates  to  the  cause  and  method  of  evolution, 
is  more  nearly  solvable  and  has  been  the  subject  of  intense  interest 
and  close  study  for  more  than  a  century.  Indeed,  it  was  our 
inability  to  explain  why  and  how  evolution  might  have  taken 


EVOLUTION  235 

place  that  for  so  long  prevented  a  general  acceptance  of  the  belief 
that  it  had  i-eally  occurred  at  all.  Although  much  progress  has 
been  made  in  this  field  of  inquiry,  we  are  still  far  from  a  complete 
and  convincing  solution  of  the  many  problems  which  the  study  of 
evolution  has  raised. 

Lamarck's  Theory. — The  first  modern  theory  which  attempted 
to  explain  evolution  was  put  forward  by  the  great  French  biologist 
Lamarck  about  1809.  He  was  much  impressed  by  the  profound 
effect  which  the  environment  produces  and  noted  many  instances 
of  its  operation,  such  as  the  vigor  and  luxuriance  of  plants  grow- 
ing in  rich  soil  as  contrasted  with  their  stunted  growth  where  the 
soil  is  poor  (Fig.  121).  In  the  cases  of  several  ''amphibious" 
species  he  observed  that  leaves  produced  when  the  plant  is  grow- 
ing under  water  are  very  different  from  those  formed  in  the  air 
(Fig.  81).  He  also  observed  the  great  structural  changes  which 
are  brought  about,  in  animals,  by  the  use  or  the  disuse  of  an 
organ.  Lamarck  beheved  that  organisms  possess  the  ability  to 
react  to  their  environment  advantageously  and  to  modify  their 
structures  and  functions  in  such  a  way  that  success  under  a 
changing  environment  will  be  attained.  He  never  questioned 
that  all  the  variations  which  he  noted  were  directly  transmitted 
to  offspring  by  heredity,  and  he  thus  pictured  the  races  of  plants 
and  animals  as  being  pushed  along  the  evolutionary  road  by 
environmental  forces. 

Lamarck's  explanation,  although  very  attractive  and  plausible 
in  certain  respects,  has  never  won  wide  acceptance.  Biologists 
have  as  a  rule  not  been  willing  to  admit  that  an  organism  has 
any  innate  ability  to  guide  its  reactions  into  a  favorable  course. 
The  theory  has  suffered  still  more  from  the  absence,  after  inten- 
sive search,  of  any  very  conclusive  evidence  that  "acquired" 
characters,  such  as  those  produced  by  the  environment,  are  ever 
inherited.  There  are  a  few  biologists  today  who  believe  that  in 
certain  of^its  features  Lamarck's  theory  comes  nearer  to  explain- 
ing the  true  method  of  evolution  than  any  other  yet  suggested,  but 
as  a  whole  it  receives  little  support. 

Darwin's  Theory.  "Natural  Selection." — The  most  notable 
attempt  to  solve  the  riddle  of  evolution  is  the  theory  of  Natural 
Selection  put  forward  in  1859  by  Charles  Darwin  (Fig.  134)  in 
"The  Origin  of  Species",  a  book  which  has  had  a  very  great  influ- 
ence on  all  human  thinking.  The  effect  of  this  theory  in  render- 
ing the  whole  process  of  evolution  plausible  and  understandable 


236  BOTANY:  PRINCIPLES  AND  PROBLEMS 

was  the  chief  factor  in  convincing  scientific  men  of  the  truth 
of  the  evokitionary  theory  in  general;  and  whatever  we  may  think 
today  of  the  merits  of  some  of  Darwin's  hypotheses,  we  recognize 
that  the  thoroughness  of  his  scientific  work  and  its  revolutionary 
effect  on  all  lines  of  biological  thought  entitle  him  to  rank  as  the 
first  great  evolutionist. 

Darwin  based  the  theory  of  Natural  Selection  upon  three  main 
facts:  Variations  and  their  inheritance;  over-production  of  ofT- 


FiG.   134.— Charles  Darwin,  1809-1882. 

spring  with  the  consequent  "struggle  for  existence";  and  the 
"survival  of  the  fittest." 

He  was  vividly  impressed  by  the  occurrence  of  variations  in  all 
animals  and  plants  and  studied  them  carefully,  endeavoring 
to  discover  their  causes.  Like  other  scientific  men  of  his  day, 
Darwin  did  not  clearly  understand  the  mechanism  and  laws  of 
inheritance.  He  believed,  at  least  in  his  earlier  work,  that 
"acquired"  characters  may  be  transmitted  to'offspring,  but  this 
belief  did  not  form  the  essential  basis  for  his  theory,  as  it  did  for 
that  of  Lamarck.  The  main  fact  which  he  emphasized  was  that 
variations  in  all  directions  are  exceedingly  abundant  and  that  in 
many  cases  they  are  certainly  transmitted  by  inheritance  to  the 
offspring. 

The  overproduction  of  progeny  in  plants  and  animals  forms  the 
next  step  in  the  theory.     If  all  seeds  which  are  produced  were  to 


EVOLUTION  237 

grow  and  if  all  animal  young  were  to  mature,  there  would  soon  be 
no  food  or  room  for  them  on  the  earth's  surface.  Only  a  small 
fraction  can  possibly  live  to  maturity.  As  a  result,  argues  Darwin , 
there  must  necessarily  be  a  terrific  life-and-death  competition, 
a  "struggle  for  existence",  in  which  the  few  survive  and  the  many 
perish. 

Finally,  those  individuals  in  this  struggle  which  possess  any 
advantage  in  structure  or  in  function  over  their  fellows,  even 
if  this  advantage  is  a  very  small  one,  will  evidently  have  the  best 
chance  to  succeed  and  survive.  Of  the  manifold  variations  which 
plants  and  animals  display,  some  will  naturally  be  helpful 
and  some  harmful,  and  those  fortunate  individuals  which  vary  in 
the  right  direction  will  survive  and  transmit  their  advantageous 
characters  to  their  offspring.  The  others  will  perish  and  leave 
no  descendants.  Through  this  "survival  of  the  fittest"  the  race 
tends  to  change  steadily  and  to  progress  toward  a  type  which  is 
better  and  better  adapted  to  the  conditions  under  which  it  is 
living,  and  also  to  develop  new  types  which  can  successfully 
invade  new  environments.  Darwin  named  this  process  Natural 
Selection  from  analogy  to  the  artificial  selection  long  practiced 
by  man  with  his  domestic  animals  and  plants,  by  which  he  has 
caused  such  great  changes  merely  by  selecting,  for  breeding 
stock  or  for  seed  production,  those  individuals  which  varied  in 
such  a  way  as  best  to  meet  his  requirements. 

Objections  to  the  Theory. — In  support  of  his  theory  Darwin 
brought  forth  a  wealth  of  evidence  so  convincing  that  it  won  very 
wide  acceptance.  As  knowledge  has  advanced,  however,  various 
objections  to  it  have  persistently  been  raised.  Why  is  an  advan- 
tageous character  appearing  in  only  one  individual  not  lost,  by 
"swamping",  in  crosses  between  this  individual  and  the  rest  of 
the  population?  Why  do  so  many  structures  exist  which  are  not 
evidently  helpful  in  survival?  Why  are  many  species  separated 
by  differences  so  small  that  it  is  hard  to  believe  that  they  are  life- 
and-death  differences?  What  causes  the  persistence  and  survival 
of  early  steps  in  the  development  of  a  structure,  before  it  has 
become  perfected  and  useful  to  the  organism  ?  If  variations  occur 
at  random,  as  Darwin  supposed,  how  does  it  happen  that  a  complex 
and  highly  coordinated  structure  could  develop,  since  its  produc- 
tion would  require  innumerable  variations  of  just  the  right  degree, 
in  just  the  right  place,  and  at  just  the  right  time?     Why  have  we 


238  BOTANY;  PRINCIPLES  AND  PROBLEMS 

never  been  able  to  produce  by  artificial  selection  a  group  of 
individuals  which  could  clearly  be  regarded  as  a  new  species? 

These  and  other  objections  have  been  answered  in  whole  or 
in  part  by  Darwinians,  but  they  are  still  of  sufficient  weight  to 
convince  most  biologists  that  the  theory  just  as  Darwin  left  it 
cannot  well  be  maintained  today.  No  doubt  natural  selection 
eliminates  vast  numbers  of  obviously  unfit  individuals,  but  that 
it  has  been  the  most  important  factor  in  producing  new  forms, 
and  thus  in  directing  evolutionary  progress,  is  now  rather 
generally  doubted. 

De  Vries's  Theory. — Another  attempt  to  explain  the  cause  and 
method  of  evolution  has  been  made  in  recent  years  by  the  Dutch 
botanist  de  Vries,  who  believes  that  the  small  and  almost  imper- 
ceptible variations,  regarded  as  most  important  by  Darwin,  are 
merely  "fluctuations"  around  the  normal  type  which  have  been 
produced  by  the  environment  and  are  therefore  not  inheritable. 
The  real  variations  which  lead  to  evolutionary  change,  according 
to  de  Vries,  are  the  mutations.  These  are  permanent  and 
inheritable,  and  are  often  large  and  conspicuous.  The  founder 
of  the  Mutation  Theory  thus  looks  upon  organic  nature  as  advanc- 
ing by  distinct  and  usually  rather  long  steps  rather  than  by  an 
almost  infinite  number  of  small  ones. 

Although  de  Vries  recognizes  the  importance  of  natural 
selection  in  evolution,  his  theory  has  certain  advantages  over 
that  put  forward  by  Darwin.  If  complex  new  characters  and 
even  new  varieties  and  species  can  arise  by  one  or  even  a  few 
steps,  the  problem  of  the  preservation  of  the  early  stages  in  the 
development  of  a  useful  structure  is  partly  solved,  and  the  exis- 
tence side  by  side  of  distinct  but  very  similar  species  is  explained. 
The  length  of  time  necessary  for  evolution  is  also  reduced.  Many 
of  the  objections  which  have  been  urged  against  Darwin's  theory, 
however,  apply  with  equal  force  to  that  of  de  Vries,  and  although 
the  latter  has  taught  us  the  necessity  of  distinguishing  sharply 
between  inheritable  and  non-inheritable  characters  in  evolution, 
it  has  not  been  accepted  as  a  complete  solution  of  the  problem. 

Orthogenesis. — These  various  theories  lack  a  convincing 
explanation  of  the  progressive  appearance  of  new  characters  and 
their  harmonious  incorporation  into  the  organism.  The  environ- 
ment evidently  cannot  produce  them,  and  it  seems  unlikely 
that  mere  random  variations,  whether  large  or  small,  would  be 
any  more  successful.     In  view  of  all  this,  some  biologists  have 


EVOLUTION  239 

turned  to  the  organism  itself  to  discover  the  directive  factor  in 
the  production  of  new  forms.  They  beheve  that  variation  is  not 
a  random  process  but  that  in  any  given  species,  or  succession 
of  individuals,  the  variations  tend  always  to  be  of  a  certain 
particular  sort,  characteristic  of  that  species,  and  that  the  species 
consequently  undergoes  progressive  change  in  a  definite  direction. 
The  advocates  of  such  a  theory  of  Orthogenesis,  or  internally 
directed  evolution,  believe  that  evolutionary  change  is  due  to  the 
unfolding  of  certain  tendencies  in  the  protoplasm  of  the  plant 
or  animal,  and  is  not  forced  upon  the  organism  from  without. 
They  recognize  the  importance  of  natural  selection  in  eliminating 
the  radically  unfit,  but  believe  this  agency  quite  unable  to  create 
anything  new  or  to  produce  the  organic  world  as  we  know  it 
today. 

It  must  be  admitted  that  as  yet  we  do  not  fully  understand  the 
manner  in  which  evolution  has  taken  place  and  the  factors  which 
have  been  responsible  for  it.  In  the  past  there  has  been  perhaps 
too  much  unsupported  speculation  on  the  problem  and  too  little 
pursuit  of  facts.  The  present  intensive  experimental  study 
of  heredity,  physiology,  cytology  and  morphogenesis  will,  it  is 
to  be  hoped,  provide  us  with  a  fund  of  information  wherewith  we 
may  attack  this  central  problem  of  biology. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

605.  Why  is  it  that  we  do  not  regard  new  strains  of  corn,  apples,  and 
similar  plants  as  new  species? 

606.  AVhy  does  the  record  of  plant  and  animal  evolution  given  us 
by  fossils  have  such  large  gaps  in  it? 

607.  What  types  of  plants  and  animals  would  be  most  likely  to  be 
preserved  as  fossils? 

608.  Characters  which  are  apparently  of  the  least  functional  impor- 
tance to  the  plant  are  often  most  constant  throughout  large  plant 
groups  and  therefore  very  valuable  in  plant  classification.     Explain. 

609.  Most  of  the  members  of  the  Figwort  family  have  four  stamens 
in  the  flower,  but  there  is  often  the  rudiment  of  a  fifth.  The  families 
most  nearly  related  to  the  Figworts  have  five  stamens.  What  can  you 
infer  from  these  facts  as  to  tlie  evolution  of  the  flower  in  the  Figworts? 

610.  The  plant  population  of  an  isolated  island  or  island  group  iu  the 
ocean  (such  as  the  Hawaiian  Islands)  is  composed  very  largely  of  species 


240  BOTANY:  PRINCIPLES  AND  PROBLEMS 

which  are  found  nowhere  in  the  world  except  on  that  particular  island 
or  island  group.     Explain. 

611.  The  potato,  tomato,  tobacco  and  various  other  agricultural 
plants  which  are  now  grown  in  Europe  were  introduced  there  from 
America  after  the  discovery  of  the  New  World.  What  can  you  conclude 
as  to  their  evolutionary  history? 

612.  Torreya,  a  genus  of  coniferous  trees,  is  represented  by  two  species 
in  China,  one  in  California,  and  another  in  the  southeastern  United 
States,  but  is  found  nowhere  else  in  the  world.  What  conclusions  can 
you  draw  from  these  facts  as  to  the  past  history  of  this  genus? 

613.  In  the  Galapagos  islands  very  many  of  the  plant  species  are 
confined  to  this  group  of  islands,  but  most  of  the  genera  are  the  same  as 
those  found  on  the  adjacent  coast  of  South  America.  In  Hawaii,  the 
species  are  not  only  distinctive  of  the  islands  but  many  of  the  genera  also 
are  found  nowhere  else.  Which  of  these  two  island  groups  on  the 
basis  of  these  facts,  do  you  believe  has  been  isolated  the  longer  and 
the  more  effectively?     Why? 

614.  Plants  living  in  arid  or  desert  regions  usually  have  small  and 
leathery  leaves  and  deep  root  systems,  in  contrast  to  plants  living  in 
regions  of  more  abundant  moisture.  How  would  Lamarck  explain 
this?     How  would  Darwin? 

615.  Most  plant  species  which  are  very  common  belong  to  genera 
which  are  larger  than  the  average  in  number  of  species.     Explain. 

616.  Darwin  noted  that  species  belonging  to  large  genera  were  usually 
more  variable  than  species  belonging  to  small  genera.     Explain. 

617.  Which  species  is  apt  to  be  more  successful,  do  you  think,  a 
relatively  variable  one  or  a  relatively  constant  one?     Why? 

618.  Which  do  you  believe  would  change  more  rapidl}^  in  evolution, 
a  species  which  is  always  cross-fertilized  or  one  which  is  always  self- 
fertilized?     Why? 

619.  State  at  least  five  advantages  which  one  plant  species  might  have 
over   another   which  would  make  it  more  widespread  and  successful. 

620.  Give  an  example  of  a  physical  barrier  to  plant  distribution; 
of  a  "biological"  barrier. 

621.  Is  competition  generally  keener  between  two  individuals  of  the 
same  species  or  between  two  individuals  belonging  to  different  species? 
Explain. 

622.  Is  competition  apt  to  be  keener  between  closely  related  or 
between  distantly  related  species?     Why? 


EVOLUTION  241 

623.  In  most  cases,  individual  plants  may  be  assigned  to  very  definite 
species,  and  between  these  species  transitional  individuals  are  rarely  or 
never  found.  If  species  have  been  developed  through  a  gradual  evolu- 
tion, why  are  such  transitional  forms  absent? 

624.  What  characteristics  must  a  successful  weed  possess? 

625.  A  weed  introduced  into  a  new  region  often  becomes  more  wide- 
spread and  successful  there  than  in  its  native  land.     Explain. 

626.  The  chestnut  bark  fungus,  introduced  some  years  ago  into  the 
United  States,  has  exterminated  all  the  native  American  chestnut  trees 
over  wide  areas.  In  China,  its  native  home,  the  species  of  chestnut  are 
almost  immune  to  its  attack.  How  do  you  explain  this  difference 
between  American  and  Chinese  chestnut  trees? 

627.  Some  species  of  plants  produce  comparatively  few  seeds  but  are 
just  as  successful  as  others  which  produce  a  great  many  more.     Explain. 

628.  During  the  glacial  invasion,  the  vegetation  of  the  northern 
United  States  was  obUged  to  migrate  hundreds  of  miles  south  of  its 
original  range,  and  as  the  ice  retreated  it  migrated  northward  again. 
Doubtless  many  plant  species  were  exterminated  during  these  changes. 
What  characteristics  should  a  plant  species  possess  to  survive  such  a 
migration  successfully? 

629.  Name  at  least  five  different  causes  which  might  lead  to  the 
extinction  of  a  plant  species. 

630.  Why  is  it  that  all  ancient  and  primitive  types  of  plants  have  not 
been  exterminated  by  the  competition  of  those  which  have  been  more 
recently  evolved? 

631.  A  highly  specialized  and  complex  plant  species  is  sometimes  far 
less  successful  than  one  which  is  much  simpler  and  more  ancient  in  type. 
Compare,  for  example,  our  common  bracken  fern,  which  thrives  over 
almost  all  the  world,  with  many  of  our  orchids,  which  are  often  rare 
and  have  very  limited  ranges.     How  do  you  explain  this? 

632.  In  the  evolutionary  history  of  many  groups  of  animals  and  plants, 
as  shown  by  their  fossils,  there  is  a  gradual  change  from  the  simple 
and  primitive  members  to  those  which  are  progressively  more  and  more 
complex  and  abundant;  but  when  a  very  high  degree  of  specialization  has 
arrived,  the  group  suddenly  becomes  extinct.     How  do  you  explain  this? 

633.  Primitive  and  ancient  types  of  animals  and  plants  are  most  com- 
mon in  comparatively  isolated  regions.     Why? 

16 


242  BOTANY:  PRINCIPLES  AND  PROBLEMS 

634.  Do  you  think  that  evohitionary  change  would  take  place  more 
rapidly  in  a  region  freely  exposed  to  immigration  from  without,  or  in  a 
comparatively  isolated  region?     Why? 

635.  The  great  land  mass  of  Europe  and  Asia  is  believed  to  have  been 
the  center  of  evolution  for  many  types  of  animals  and  plants  now  found 
in  other  parts  of  the  world.     Why? 

636.  Are  the  most  widely  spread  plant  species  the  oldest,  do  you 
think?     Explain. 

637.  In  consequence  of  the  "struggle  for  existence"  and  the  "sur- 
vival of  the  fittest,"  why  is  it  that  in  a  given  locality  one  species  does 
not  exterminate  all  the  others  and  compose  the  entire  vegetation? 

638.  Name  a  few  of  the  changes  in  the  natural  vegetation  of  the 
world  which  have  been  brought  about  by  civilization. 

REFERENCE  PROBLEMS 

102.  Give  an  example  of  a  new  variety  of  cultivated  plant  which  has 
recently  been  developed  by  plant  breeders. 

103.  About  how  long  do  geologists  estimate  that  life  has  existed  on  the 
earth? 

104.  What  are  the  great  geological  periods  into  which  the  ancient  history 
of  the  earth  has  been  divided  by  geologists? 

105.  Summarize  the  life  and  work  of  Lamarck  and  state  his  important 
contributions  to  botany. 

106.  Summarize  the  life  and  work  of  de  Vries  and  state  his  important 
contributions  to  botany. 

107.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Evolution  Fossil  Orthogenesis 


CHAPTER  XIII 

THE  PLANT  KINGDOM 

Through  a  period  reaching  back  into  the  past  for  milKons  of 
years,  such  a  long  time  that  the  entire  span  covered  by  human 
history  seems  almost  negligible  beside  it,  the  evolutionary 
advance  of  the  plant  kingdom  has  gone  slowly  but  steadily 
onward.  We  may  still  be  uncertain  as  to  the  causes  which  lie 
behind  this  tremendous  progressive  movement,  but  its  results 
are  manifest  in  the  scores  of  thousands  of  plant  species  with 
which  the  earth  is  covered  today.  Those  of  us  who  are  familiar 
with  the  vegetation  of  the  temperate  zone,  thriving  and  vigorous 
though  this  may  be,  can  have  little  idea  of  the  luxuriance  and 
variety  of  plant  life  exhibited  in  tropical  and  subtropical  regions. 
In  New  England  there  are  about  4,000  species  of  seed  plants, 
but  probably  50,000  occur  in  Brazil,  and  in  the  whole  world  there 
have  already  been  described  a  vast  array  of  almost  250,000  species. 
Nor  is  our  knowledge  by  any  means  complete.  Although  for  the 
past  three  hundred  years  botanical  exploration  has  been  active  in 
all  parts  of  the  globe,  the  discovery  ofnew  species  is  still  constantly 
being  reported.  It  is  the  seed  plants  which  constitute  the 
dominant  and  conspicuous  part  of  this  multitudinous  vegetation, 
and  in  the  present  volume  we  have  confined  our  attention  almost 
exclusively  to  them,  but  we  should  remember  that  the  plant  king- 
dom includes  a  host  of  lower  and  simpler  members.  Of  the  ferns 
and  their  allies  we  now  know  more  than  4,500  species,  and  in  many 
parts  of  the  earth  they  are  an  important  element  in  the  plant 
population.  Of  the  liverworts  and  mosses  there  are  some  16,000 
species,  and  these  are  far  exceeded  by  still  lower  forms,  the  fungi 
with  60,000  species  and  the  algae  with  20,000.  In  all  these  groups, 
exploration  and  critical  study  are  yearly  adding  many  new  forms 
and  it  is  safe  to  estimate  that,  were  our  knowledge  complete,  we 
should  be  able  to  recognize  not  less  than  300,000  species  of 
plants.  The  day  has  passed  when  any  one  botanist  can  hope  to 
become  familiar  with  more  than  a  small  portion  of  the  flora  of 
the  globe. 

243 


244  BOTANY:  PRINCIPLES  AND  PROBLEMS 

That  record  of  remote  events  which  has  survived  to  our  day  in 
the  form  of  fossil  plants  makes  it  clear  that  the  panorama  of  the 
earth's  vegetation  has  repeatedly  changed,  that  group  after  group 
has  arisen,  flourished  and  disappeared,  and  that  thousands  of 
species  have  evolved  only  to  become  extinct.  Plants  of  today 
are  the  product  of  a  long  period  of  evolutionary  development, 
and  to  understand  the  vegetable  kingdom  and  its  relationships 
we  must  therefore  know  something  of  the  main  events  in  its 
history. 

Plants  and  Animals. — Plants  and  animals  constitute  the  two 
great  branches  of  the  organic  world.  In  their  lowest  represen- 
tatives they  are  often  hard  to  distinguish  from  one  another,  and 
certain  simple  forms  exist  which  clearly  combine  the  characters 
of  both  kingdoms.  The  earhest  of  living  things  would  perhaps 
have  been  difficult  for  us  to  classify,  but  as  evolution  progressed, 
animals  and  plants  became  clearly  distinguishable  through  the 
development  among  their  members  of  certain  characteristic 
traits.  The  animal  tends  to  be  motile,  to  ingest  its  food  through 
a  mouth,  and  to  depend  on  other  organisms  as  sources  of  food 
supply;  the  plant,  to  be  stationary,  to  absorb  its  nutrient  mate- 
rials in  solution  over  a  considerable  area  of  the  body,  and  (except 
in  the  fungi  and  a  few  others)  to  manufacture  its  own  food  from 
simple  inorganic  substances.  Many  other  differences  in  struc- 
ture and  function  are  associated  with  these  fundamental  ones. 

Forward  Steps  in  Plant  Evolution. — During  the  divergent 
history  of  the  two  great  groups,  certain  notable  events  took  place 
in  each  with  which  the  student  of  biology  should  become  familiar. 
Before  discussing  the  classification  of  the  plant  kingdom  which 
follows  in  the  succeeding  chapters,  we  shall  therefore  consider 
briefly  a  few  of  the  important  steps  which  have  marked  its 
development. 

The  causes  which  led  to  the  appearance  on  our  earth  of  the 
first  living  things,  and  the  characteristics  which  these  primitive 
organisms  displayed,  are  buried  so  deeply  in  antiquity  that  we 
shall  probably  never  discover  them.  There  is  good  reason  to 
beheve,  however,  that  among  the  earliest  of  all  plants,  thriving 
in  the  warm  primeval  seas,  were  simple,  single-celled  forms  which 
multiplied  by  simple  division  or  fission,  possessed  chlorophyll  or  a 
similar  substance,  and  in  their  general  characters  were  not 
greatly  unlike  some  of  the  simplest  of  our  living  algae.  For 
ages  they  doubtless  were  the  only  vegetable  life  on  the  globe. 


THE  PLANT  KINGDOM 


245 


1.  The  Multicellular  Plant. — The  first  great  forward  step 
which,  like  all  first  steps,  was  probably  a  long  time  in  being 
accomplished,  consisted  of  the  union  of  these  simple  cells  into 
colonies  (Fig.  135).  The  two  daughter-cells  formed  at  a  division 
remained  attached  to  one  another 
instead  of  separating,  and  thus 
arose  small  cell-groups  or  aggrega- 
tions such  as  we  still  may  see 
among  the  lowest  algae.  The 
individual  cells  forming  these 
groups  might  cohere  variously — in 
spherical  masses,  in  threads,  or  in 
sheets.  Through  a  still  more  in-  f 
timate  union  between  their  members 
these  cell  colonies  gradually  de- 
veloped into  definite  multicellular 
plants,  various  in  size  and  shape 
and  probably  much  like  some  of  the 
simpler  seaweeds  of    today.     The 


A  C 

Fig.   135.  Fig.   136. 

Fig.  135. — The  beginnings  of  a  multicellular  plant.  A  simple  alga,  Pleuro- 
coccus,  in  which  the  plant  body  is  commonly  a  single  cell,  but  in  which  the 
daughter  cells  following  cell  di\dsion  may  remain  united  in  very  simple  colonies. 

Fig.  136. — The  beginnings  of  differentiation.  A  thread-like  or  filamentous 
alga,  Oedogonium,  in  which  the  cells  are  no  longer  all  alike  but  have  begun  to  be 
differentiated.  One  is  modified  as  a  holdfast  (C),  others  as  male  sexual  organs 
(a)  and  others  as  female  sexual  organs  (o). 

way  was  thus  opened  for  the  production  of  those  very  large  and 
complex  plant  bodies  with  which  we  are  most  familiar. 

2.  Differentiation. — The  evolution  of  the  many-celled  plant 
was  soon  followed  l>y  another  and  equally  important  advance,  the 
beginning  of  differentiation  (Fig.  136).  The  primitive  single  cell 
performed  all  the  functions  which  we  now  associate  with  the 
entire  plant,  such  as  absorption,  photosynthesis,  and  reproduc- 
tion.    Soon  after  the  multicellular  individual  had  arisen,  how- 


246  BOTANY:  PRINCIPLES  AND  PROBLEMS 

ever,  there  began  to  appear  within  it  the  same  tendency  which 
manifests  itself  in  the  evolution  of  a  human  society,  the  "division 
of  labor".  Instead  of  the  primitive  condition  in  which  all  the 
individual  cells  carry  on  all  the  functions,  certain  cells  became 
specialists,  some  of  them  gradually  assuming  the  performance  of 
one  function  and  some  of  another,  and  acquiring  through  this 
specialization  a  more  or  less  conspicuous  modification  in  struc- 
ture. The  first  activity  of  plants  to  be  thus  locahzed  was  prob- 
ably reproduction.  Instead  of  a  condition  where  every  cell 
divided  and  gave  rise  to  new  individuals,  certain  ones  were  set 
apart  to  produce  specialized  reproductive  cells  or  spores,  provided 
with  means  of  locomotion  or  other  facilities  which  made  them 
particularly  well  adapted  to  establish  a  multitude  of  new  and 
widely  scattered  plants.  This  process  of  differentiation  has  stead- 
ily progressed  during  the  evolution  of  the  plant  kingdom  and  has 
resulted  in  the  marvelously  complex  individuals  which  we  have 
studied  among  the  seed  plants.  Here  the  various  functions 
have  organs  devoted  to  their  performance  and  in  these  the  cells, 
far  from  being  uniform,  are  grouped  in  definite  and  highly 
specialized  tissues,  each  of  which  plays  its  particular  part  in  the 
life  of  the  whole.  Differentiation  has  made  possible  the  existence 
of  the  higher  plants,  and  is  one  aspect  of  that  phenomenon  of 
organization  or  regulation  to  which  we  have  so  often  called 
attention. 

3.  Sexual  Reproduction. — Another  important  step  in  the 
history  of  the  plant  kingdom  involved  the  method  by  which 
reproduction  took  place.  In  the  earliest  plants,  this  process  was 
accomplished  merely  by  a  division  of  the  cell  into  two.  In 
forms  a  little  more  advanced,  special  cells  became  differentiated, 
each  of  which  was  able  to  produce  a  new  plant.  Following  this 
stage,  the  type  of  reproduction  which  we  know  as  sexual  probably 
made  its  appearance.  The  essential  feature  of  this  method  is 
the  fusion  of  two  cells  into  one  and  the  subsequent  development 
therefrom  of  a  new  individual  (Fig.  137).  The  cells  which  thus 
unite  are  called  sexual  cells  or  gametes,  and  the  product  of  their 
union,  the  zygote.  In  early  plants,  the  gametes  were  probably 
nothing  more  than  the  ordinary  non-sexual  reproductive  cells 
which  had  assumed  this  additional  function;  and  in  some  of  the 
algae  today  we  find  cells  of  this  sort,  which  may  reproduce  the 
plant  either  sexually  or  asexually.  Soon  the  gametes  became 
clearly  distinct,  however,  and  asexual  reproduction  was  often 


THE  PLANT  KINGDOM 


247 


given  up,  or  resorted  to  only  under  special  conditions.  The 
sexual  cells  themselves  became  further  differentiated  into  two 
sorts — small,  active  ?na/e  gametes  or  sperms  and  larger,  non- 
motile  female  gametes  or  eggs,  a  condition  which  now  accompanies 
sexuality  so  commonly  that  instances  of  equal  gametes  are  corn- 


el B 

Fig.  137. — The  beginnings  of  sexuality.  A  very  simple  alga,  Chlamydomonas, 
(A)  in  which  two  cells  may  be  differentiated  as  gametes  and  unite  with  each  other. 
From  their  union  a  group  of  new  individuals  arises.  In  the  case  of  fertilization 
here  illustrated  {B)  the  gametes  are  slightly  different  in  size,  foreshadowing  the 
development  of  male  and  female  gametes. 


paratively  rare.  The  causes  which  led  to  the  development  of 
sexual  reproduction  are  unknown,  but  the  process  is  so  nearly 
universal,  not  only  among  plants  but  throughout  the  animal 
kingdom,  that  we  are  forced  to  believe  it  must  have  some  special 
significance.  There  is  evidence  that  sexual  fusion  results  in 
increased  vigor,  particularly  when  the  two  gametes  come  from 
different  individuals;  but  we  also  know  man}-  plants  which  may 
reproduce  indefinitely  by  various  asexual  processes  without  evi- 
dent   loss    in   vitality.     However   that  may  be,  a  considerable 


248 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


THE  PLANT  KINGDOM  249 

proportion  of  the  activities  of  plants  seems  to  be  devoted  to  the 
successful  accomplishment  of  the  sexual  process. 

4.  Alternation  of  Generations. — The  three  steps  in  plant 
evolution  which  we  have  mentioned — the  appearance  of  the 
multicellular  individual,  of  functional  and  structural  differentia- 
tion, and  of  sexual  reproduction — occurred  also  among  animals. 
The  fourth  belongs  exclusively  to  the  plant  kingdom.  It  is  the 
evolution  of  that  remarkable  double  life-cycle  which  we  know  as 
the  "alternation  of  generations"  (Fig.  138),  a  process  simple 
enough  in  its  underlying  principle  and  in  its  expression  among 
the  lower  plants,  but  which  in  the  higher  groups  leads  to  such 
complexities  than  an  understanding  of  the  process  of  repro- 
duction is  more  difficult  in  the  vegetable  kingdom  than  among 
animals. 

A.  In  Thallophytes. — In  most  members  of  that  great  and  primi- 
tive group  the  Thallophytes,  which  include  the  algae  and  the 
fungi  and  which  is  the  lowest  of  the  four  main  divisions  of 
the  plant  kingdom,  a  fertilized  egg  gives  rise  directly  to  a  single 
new  individual,  just  as  is  the  case  among  animals.  Beginning 
rather  obscurely  among  some  of  the  higher  Thallophytes  and 
reaching  its  full  expression  first  in  the  liverworts  and  mosses,  we 
find  a  modification  of  this  simple  and  direct  method.  The 
fertihzed  egg,  instead  of  producing  a  new  plant  like  the  parent, 
divides  into  a  group  of  cells  which  separate  and  are  liberated 
as  non-sexual  spores.  Each  spore  may  now  give  rise  to  a 
new  plant.  In  this  way  a  single  sexual  union  produces  a 
whole  group  of  new  plants  instead  of  one;  and  it  is  this  ad- 
vantage, seized  upon  and  perfected  by  early  members  of  the 
plant  kingdom,  which  probably  led  to  the  characteristic  "alter- 
nating" systems  of  reproduction  in  all  plants  above  the  Thallo- 
phytes— a  fertilized  egg  producing  a  group  of  spores,  each  of 
which  grows  into  a  sexual  plant,  in  which  fertilization  again 
takes  place. 

B.  In  Bryophytes. — Such  a  simple  condition  as  this,  however, 
was  evidently  soon  outgrown  and  now  occurs  among  only  a  few 
lowly  forms.  The  rapidly  enlarging  group  of  cells  which  had 
their  origin  from  the  fertilized  egg  soon  began  to  produce  some- 
thing more  than  a  mass  of  spores.  The  outermost  cells  became 
differentiated    into    a   protecting   wall,    and  the  spore-case  or 


250  BOTANY:  PRINCIPLES  AND  PROBLEMS 

sporangium  was  thus  formed.  The  cells  at  the  base  of  this 
structure  further  developed  into  a  support  or  stalk  which  length- 
ened and  carried  the  sporangium  up  into  the  air,  whence  the  spores 
might  readily  be  dispersed.  This  general  situation,  where  the 
sexual  plant  produces  a  spore-case  borne  on  a  well-developed 
stalk,  is  characteristic  of  the  second  main  division  of  the  plant 
kingdom,  the  Bryophytes,  which  include  the  liverworts  and  the 
mosses. 

C.  In  Pteridophytes. — The  next  stage  in  the  development  of 
the  "alternating"  life  cycle  involved  a  radical  change,  and  since 
the  plants  in  which  this  change  took  place  have  long  since 
perished  and  have  left  no  descendants  to  our  day,  we  can  only 
surmise  as  to  what  actually  happened.  The  sporangium  and  its 
related  structures  underwent  a  remarkable  transformation, 
developing  chlorophyll-bearing  leaves  which  manufactured  its 
food  supply,  and  finally  sending  forth  roots  into  the  soil  and  thus 
establishing  a  new  and  entirely  independent  individual.  Instead 
of  producing  but  one  sporangium,  this  now  gave  rise  to  many. 
In  short,  a  spore-bearing,  non-sexual  plant,  the  sporophyte, 
had  been  evolved,  entirely  distinct  from,  and  independent  of,  the 
older  gamete-bearing,  sexual  plant,  which  is  known  as  the 
gametophyte.  *  This  is  the  situation  as  we  meet  it  in  the  third  great 
division  of  the  plant  kingdom,  the  Pteridophytes,  which  include 
the  ferns,  club  mosses,  horse-tails,  and  their  allies  (Fig.  138) .  Here 
the  dominant,  conspicuous  plant,  with  which  we  are  most  famihar, 
is  the  sporophyte.  It  produces  thousands  of  sporangia  and 
perhaps  milhons  of  spores,  but  sexual  organs  and  gametes  are 
entirely  absent  from  it.  It  has  evolved  from  the  primitive  moss 
sporangium  and  its  associated  organs.  The  gametophyte, 
however — the  structure  which  corresponds  to  the  moss  plant — is 
small,  inconspicuous  and  often  hard  to  discover.  It  possesses  no 
true  roots  or  leaves  but  bears  the  all-important  sexual  cells. 
Here  the  fusion  between  gametes  takes  place,  and  from  a  fertil- 
ized egg  in  one  of  these  inconspicuous  gametophytes  is  developed 
a  young  spore-bearing  plant.  This  soon  develops  leaves  and 
roots  and  becomes  independent  of  the  parent  gametophyte, 
which  then  withers  away.     Each  of  the  many  spores  borne  by 

*  The  terms  sporophyte  and  gaynetophyte  are  also  used  among  Bryophytes, 
the  former  being  applied  to  the  spore-case  and  its  related  structures  and  the 
latter  to  the  plant  itself. 


THE  PLANT  KINGDOM  251 

this  spore-plant  will  germinate  and  grow  (if  conditions  arc 
favorable)  and  will  develop  in  turn  into  a  new  gamete-plant, 
which  will  produce  gametes  and  effect  fertilization  as  before. 
In  such  a  life-cycle  as  this,  there  are  two  distinct,  independent 
and  alternating  plant  types  or  "generations,"  each  always 
producing  the  other.  It  was  a  study  of  conditions  in  these 
Pteridophytes  which  led  to  the  term  "alternation  of  generations", 
and  which  caused  us  to  realize  the  significance  of  the  peculiar 
methods  of  reproduction  found  in  both  the  mosses  and  the 
seed  plants. 

Another  notable  distinction  between  gametophytc  and  sporo- 
phyte  lies  in  the  number  of  chromosomes  found  in  their  cells. 
In  the  former  this  number  is  only  half  as  great  as  in  the  latter. 
The  gametophyte  really  begins  at  the  "reduction  division" 
(p.  187  ),  which  occurs  in  one  of  the  cell  divisions  preceding  the 
formation  of  the  spores,  and  ends  with  the  union  of  the  gametes 
in  fertilization,  which  restores  the  double  chromosome  number  and 
begins  the  sporophyte. 

D.  In  Sperrnatophytes. — Finally,  in  the  fourth  and  highest 
division  of  the  plant  kingdom,  the  Spermatophytes  or  seed  plants, 
the  alternation  of  generations  has  reached  a  still  further  stage  of 
specialization.  Here  the  gametophyte,  instead  of  being  an 
independent  structure,  now  remains  attached  to  and  dependent 
upon  the  sporophyte.  Furthermore  there  are  now  two  kinds  of 
spores,  the  microspores  (essentially  what  we  know  as  pollen 
grains)  which  develop  into  much  reduced  male  gametophytes, 
producing  only  male  gametes;  and  the  megaspores,  borne  in  the 
ovules,  and  developing  there  into  female  gametophytes,  producing 
only  egg  cells.*  At  maturity,  the  male  gamete  comes  down  the 
pollen  tube  and  fertilizes  the  egg  in  an  ovule.  From  this  union 
the  young  sporophyte  develops  as  the  embryo  of  the  seed,  and 
will  in  turn  grow  into  a  plant  producing  thousands  of  spores. 
In  the  seed  plants,  both  gametophytes  are  much  reduced  in  size 
and  have  so  lost  their  primitive  character  that  it  was  long 
before  they  were  recognized  as  gametophytes  at  all. 

In  the  history  of  the  plant  kingdom  we  thus  pass  from  plants 
which  are,  like  animals,  entirely  gamete-bearing  (in  the  Thallo- 

*  Microspores  and  megaspores  are  differentiated  in  a  few  of  the  higher 
Pteridophytes,  which  we  shall  later  describe.  No  seeds  are  developed  among 
these  plants,  however. 


252  BOTANY:  PRINCIPLES  AND  PROBLEMS 

phytes)  to  those  in  which  the  gamete-plant  alternates  with  a 
small,  dependent,  spore-bearing  structure,  the  primitive  sporo- 
phyte  (in  the  Bryophytes);  thence  to  forms  where  sporophjiie 
and  gametoph}^e  are  both  independent  plants  but  where  the 
former  is  now  the  large  and  conspicuous  member  (in  the  Pterido- 
phytes);  and  finally  to  those  in  which  the  gametophyte  is  the 
dependent  and  subordinate  generation,  and  where  the  only  plant 
which  we  know  as  such  is  the  spore-plant  (in  the  Spermatophytes). 
A  knowledge  of  this  progressive  development  will  not  only  aid 
us  in  understanding  the  process  of  reproduction  in  plants  but  is 
perhaps  the  best  approach  by  which  we  can  gain  a  clear  concep- 
tion of  some  of  the  important  distinctions  between  the  four  great 
divisions  of  the  plant  kingdom. 

5.  The  Invasion  of  the  Land. — The  fifth  great  forward  step  in 
plant  history  was  the  evolution  of  a  type  able  to  grow  in  the  air 
rather  than  in  the  water,  and  which  thus  made  possible  an 
invasion  of  the  dry  land  and  the  establishment  there  of  a  real 
terrestrial  vegetation.  We  have  said  that  plant  life  probably 
began  in  the  sea.  Here  also  doubtless  took  place  the  first  great 
steps  in  the  evolution  of  the  vegetable  kingdom;  and  although 
the  seas  teemed  with  life,  the  land  masses  of  our  earth  were  for  a 
very  long  period  of  time  barren  wastes,  or  at  best  covered  in 
their  damper  spots  only  with  a  scum  of  algae.  This  great 
area  was  freely  open  to  whatever  plant  pioneer  should  be  able  to 
master  the  difficulties  of  such  an  environment. 

Difficulties  of  Terrestrial  Life. — These  difficulties  were  many 
and  formidable.  First  and  most  serious  among  them  was  the 
problem  of  maintaining,  in  such  dry  surroundings,  a  sufficient 
supply  of  water  for  protoplasmic  activity.  We  have  discussed 
in  an  earlier  chapter  the  supreme  importance  of  water  in  the  life 
of  plants,  and  have  shown  how  indispensible  it  is  in  all  physiolog- 
ical processes.  When  the  whole  plant  body  is  immersed  in  water, 
as  is  the  case  in  primitive  and  lowly  forms,  an  ample  supply  of 
this  substance  is  always  at  hand.  If  a  plant  part  is  lifted  up  into 
the  air,  however,  it  is  at  once  exposed  to  the  danger  of  water- 
loss  through  evaporation,  which  will  soon  result  in  death.  This 
danger  of  drought  has  always  faced  plants  which  grow  upon  the 
land.  If  a  plant  is  to  succeed  in  such  an  arid  environment  it 
must  be  able  both  to  absorb  water  in  large  amounts  and  to  hinder 
the  loss  of  water  from  its  tissues  by  evaporation.     Since  the  soil 


THE  PLANT  KINGDOM  253 

provides  the  only  source  of  water  available  to  a  land  plant,  it  is 
evident  that  roots  or  root-hke  structures  must  be  developed  to 
penetrate  the  soil  and  absorb  water  therefrom  abundantly. 
A  successful  accomplishment  of  photosynthesis  requires  a  large 
area  of  chlorophyll  exposed  to  sunlight,  and  hence  broad  sheets 
of  chlorophyll-bearing  tissue,  "waterproofed"  to  prevent  undue 
evaporation,  must  also  be  evolved.  These  sheets  we  call  leaves. 
The  leaves  cannot  be  tdo  close  together  without  depriving  one 
another  of  the  necessary  light,  and  they  must  therefore  be  spread 
out  and  separated  in  some  way  on  an  axis  or  stem.  The  region 
where  water  is  constantly  needed  to  replace  water  loss  may  thus 
be  far  distant  from  the  region  where  it  is  absorbed,  and  a  well 
developed  conducting  system  to  carry  water  from  root  to  leaf 
must   therefore   be   differentiated   in  the   tissues   of  the   stem. 

Aside  from  the  difficulty  of  maintaining  a  sufficient  supply 
of  water,  the  land  plant  also  faces  problems  of  a  mechanical  nature. 
Owing  to  the  buoyancy  of  water,  a  plant  growing  submersed 
therein  needs  little  or  no  mechanical  support.  If  it  grows  in  the 
air,  however,  there  is  much  weight  to  be  carried  and  a  heavy 
strain  to  be  borne  by  the  stem,  especially  in  its  lower  portions. 
An  extensive  development  of  thick-walled  skeletal  and  supporting 
tissue  is  thus  necessary,  especially  in  the  stem,  if  the  plant  is  to 
be  kept  firm  and  erect. 

In  order  to  be  able  to  thrive  on  land  a  plant  must  therefore 
possess  successfully  functioning  roots  and  leaves,  and  a  stem 
able  to  serve  as  an'  efficient  means  of  conduction  and  support. 
Such  structures  are  unknown  in  the  Thallophytes,  and  these 
plants  have  therefore  never  been  able  to  invade  the  dry  land  and 
to  produce  a  true  terrestrial  vegetation,  although  they  often 
thrive  in  moist  situations  on  land  and  survive  long  periods  of  dry- 
ness in  a  dormant  state.  The  first  group  to  emerge  from  the 
water  and  develop  land-inhabiting  forms  were  probably  the 
Bryophytes  or  plants  like  them,  which  may  perhaps  be  called 
the  "amphibians"  of  the  plant  world.  They  are  best  developed 
in  moist  places,  though  a  few  are  aquatic  and  many  grow  in 
situations  which  are  dry  much  of  the  time.  Even  in  the  most 
highly  developed  mosses,  however,  the  root  system  is  very  weak 
and  consists  only  of  delicate  thread-like  rhizoids;  the  leaves  are 
small  and  very  thin,  and  the  stems  weak  and  with  little  or  no 
development  of  supporting  and  conducting  tissue.     The  mosses, 


254  BOTANY:  PRINCIPLES  AND  PROBLEMS 

therefore,  do  not  grow  more  than  a  few  centimeters  high  and 
have  never  succeeded  in  producing  a  strong  and  vigorous  land 
vegetation. 

Success  of  the  Pteridophytes. — It  is  a  different  matter  with  the 
Pteridophytes,  however.  The  dominant  generation  here,  as  we 
have  seen,  is  the  sporophyte;  and  this  new  plant  type,  at  least  in 
all  the  forms  which  have  survived  to  the  present  day,  seems  to 
be  particularly  well  adapted  to  terrestrial  life.  Here  for  the  first 
time  we  meet  with  true  roots — large,  vigorous,  much-branched 
structures,  each  terminating  in  a  mass  of  roots  hairs  and  well 
suited  for  rapid  absorption  and  strong  anchorage.  The  leaf, 
instead  of  being  a  small  and  thin  plate  of  tissue,  is  large  and 
relatively  thick.  It  has  a  well-developed  mesophyll  of  thin- 
walled  cells  and  is  provided  with  abundant  air  spaces,  the  whole 
structure  being  covered  by  a  stout  epidermis  to  cut  down  evapora- 
tion. The  necessary  passage  of  gases  between  the  outer  air  and 
the  internal  tissues  of  the  leaf  takes  place  through  characteristic 
pores  or  stomata.  The  stem  reaches  a  structural  complexity 
nowhere  exceeded  among  plants,  the  tissues  for  support  and  con- 
duction being  particularly  well  developed.  The  evolution  of  the 
true  root,  the  true  leaf,  the  stoma,  and  the  highly  differentiated 
stem  made  it  possible  for  Pteridophytes  to  produce  the  vigorous 
and  abundant  land  vegetation  which  covered  the  earth  in  ancient 
times;  and  from  this  group  have  come  the  seed  plants,  which 
form  the  bulk  of  the  terrestrial  vegetation  of  today. 

There  is  evidently  a  wide  step  between  the  mosses  on  the  one 
side  and  the  terns  on  the  other.  Transitional  forms  which  must 
have  existed  between  these  two  have  entirely  disappeared,  and 
we  can  only  guess  what  the  plants  were  like  which  connected  the 
Bryophytes  and  the  Pteridophytes,  and  what  were  the  first  steps 
in  the  evolution  of  the  well-developed  land-inhabiting  sporo- 
phytes  which  are  so  conspicuous  today.  This  successful  invasion 
of  the  dry  land  stands  out  as  one  of  the  most  important  and 
dramatic  events  in  the  history  of  the  plant  kingdom. 

6.  The  Evolution  of  the  Seed. — The  last  great  progressive 
movement  which  we  shall  consider  is  the  comparatively  recent  one 
which  carried  the  process  of  reproduction  to  a  still  higher  degree 
of  efficiency  and  resulted  in  the  development  of  that  most  perfect 
of  reproductive  structures,  the  seed. 

The   production   of   seeds   is   the   distinctive   feature   of   the 


THE  PLANT  KINGDOM  255 

Spermatophytes  or  seed  plants,  which  are  now  the  most  successful 
of  all  the  higher  plant  types.  The  spore  has  several  obvious  dis- 
advantages as  a  means  of  producing  a  new  plant,  owing  to  its 
minute  size.  Among  the  lower  plants  these  difficulties  are  parti- 
ally overcome  by  the  production  of  spores  in  huge  quantities,  but 
the  system  of  independent,  free-living  gametophytes,  developed 
from  single-celled  spores,  is  subject  to  many  difficulties  at  best. 
In  the  seed  plants,  as  in  a  few  of  the  most  advanced  Pteridophytes, 
there  are  (as  we  have  previously  noted)  two  kinds  of  spores — 
microspores  and  megaspores — which  produce  male  and  female 
gametophytes,  respectively.  The  happy  innovation  introduced 
by  these  highest  plants,  however,  was  to  retain  the  single  mega- 
spore  within  the  sporangium,  intimately  attached  to  the  mother 
plant,  where  it  germinates  into  a  much  reduced  female  gameto- 
phyte.  This  whole  structure,  with  the  addition  of  a  coat  or 
integument,  is  the  ovule.  Only  a  few  ovules,  in  comparison  with 
the  great  number  of  spores  formerly  produced,  are  borne  by  the 
plant.  The  microspores  (pollen-grains)  are  still  liberated  into 
the  air  in  great  numbers,  just  as  among  lower  plants,  but  instead 
of  falling  on  the  ground  and  germinating  there,  they  are  carried 
to  the  ovule  or  near  it,  where  each  produces  two  male  gametes, 
one  of  which  may  fertilize  an  egg. 

Not  only  have  the  seed  plants  abolished  the  delicate,  free- 
living  gametophytes,  with  all  the  consequent  dangers  and  diffi- 
culties in  the  process  of  reproduction,  but  they  have  also 
established  a  much  more  successful  method  for  insuring  the 
growth  of  the  young  plant.  The  fertilized  egg  grows  at  once 
into  the  embryo,  which  draws  the  materials  for  its  development 
directly  and  abundantly  from  the  mother  plant,  and  is  thus 
relieved  of  the  necessity  of  producing  them  by  its  own  activity. 
About  the  embryo  is  deposited  this  supply  of  concentrated  food  in 
the  form  of  endosperm.  The  growth  of  the  embryo  ceases  after 
a  young  root  and  one  or  two  primitive  leaves  have  been  formed; 
and  embryo  and  endosperm,  tightly  enclosed  in  the  integument  of 
the  ovule  which  has  now  become  very  tough  and  strong,  is  known 
as  the  seed.  This  becomes  detached  from  the  parent  plant  and 
may  remain  in  a  dormant  condition  for  a  long  time,  sometimes 
many  years;  but  on  the  occurrence  of  favorable  conditions  it 
will  germinate  and  the  embryo  within  it  will  begin  to  grow, 
bursting  its  shell,  al)S()r])iiig  the  stored  food,  sending  forth  roots 


256  BOTANY:  PRINCIPLES  AND  PROBLEMS 

and  leaves,  and  rapidly  developing  into  a  new  plant.  The  many 
advantages  of  reproduction  by  seeds  over  the  old  method  of 
wind-blown  spores  and  independent  gametophytes  are  obvious, 
and  it  is  easy  to  see  why  the  seed-plants  have  become  so  dominant 
and  successful. 

Plant  Classification. — As  the  result  of  these  slow,  progressive 
changes,  which  have  been  working  themselves  out  gradually 
through  millions  of  years,  we  see  around  us  the  plant  kingdom  of 
today,  occupying  almost  every  corner  of  surface  on  land  and  sea 
and  consisting  of  an  enormous  variety  of  species.  The  task  of 
the  science  of  taxonomy  is  not  merely  to  list  and  describe  these 
species,  but  to  classify  this  great  array  by  distinguishing  and 
bringing  together  groups  of  species  which  resemble  each  other, 
thus  reducing  our  knowledge  of  the  plant  kingdom  to  that  orderly 
arrangement  which  is  the  aim  of  all  science.  Ever  since  the  dawn 
of  botanical  study,  men  have  been  endeavoring  to  construct 
such  a  classification  and  the  results  are  very  diverse.  One  of 
the  earliest  attempts  used  the  resemblance  in  growth-habit  as 
a  basis  of  classification  and  divided  plants  into  three  groups, — 
trees,  shrubs,  and  herbs.  As  botanists  learned  more  about  the 
vegetable  kingdom,  such  crude  systems  were  seen  to  be  wholly 
inadequate,  and  resemblances  of  a  much  more  deeply-seated 
kind  began  to  be  noted,  based  on  a  larger  number  of  characteris- 
tics. Thus  the  conception  of  plant  "families"  began  to  take 
form,  and  the  Rose  family,  the  Carrot  family,  the  Legume 
family,  and  many  others  were  distinguished  and  described. 
There  were  still  wide  differences  of  opinion  as  to  what  the  groups 
should  be  and  how  they  should  be  subdivided,  and  there  were 
almost  as  many  "systems"  as  botanists.  Indeed,  on  the  theory 
which  assumed  that  all  plants  had  been  created  at  the  same  time, 
it  was  difficult  to  see  why  these  well-marked  groups  of  similar 
species  should  exist  at  all,  and  there  was  really  no  rational  foun- 
dation for  any  system  of  classification. 

The  establishment  of  the  theory  of  evolution  in  the  latter  part 
of  the  nineteenth  century,  however,  threw  a  flood  of  light  on  the 
whole  problem,  for  it  showed  that  resemblance  among  members  of 
a  plant  group  was  not  an  arbitrary  or  chance  one  but  was  due  to 
the  fact  that  all  the  members  had  descended  from  a  commoji  ancestor. 
Classification  became  thereupon  a  definite  effort  to  work  out  a 
genealogy  or  "family  tree"  for  the  plant  kingdom,  or  for  a  given 
group  within  it,  similar  in  its  type  to  that  which  we  might  construct 


THE  PLANT  KINGDOM 


257 


for  any  family  (Fig.  139).  The  problem  confronting  the  taxono- 
mist  today,  therefore,  is  not  the  recognition  of  certain  rather 
vague  "affinities"  between  plants,  but  simply  the  determination 
of  what  might  be  called  their  "blood  relationships";  and  during 
the  past  fifty  years  particular  emphasis  has  been  placed  on  the 


Ancient  Coniferous  Stock 


Fig.  139. — A  suggested  "family  tree"  for  the  Conifers.  According  to  this 
hypothesis,  the  ancient  coniferous  stock  long  ago  divided  into  two  groups,  each  of 
which  has  given  rise  to  three  modern  families  or  sub-families.  Two  of  these  (the 
Taxineae  and  Podocarpineae)  are  more  closely  related  to  each  other  than  they  are 
to  the  other  families.  Within  the  Abietineae  there  are  evidently  two  distinct 
groups  of  genera.  The  branch  stumps  represent  extinct  groups.  The  twigs  are 
genera  of  coniferous  trees  which  are  living  today.  Such  a  diagram  as  this  makes 
it  possible  to  show  graphically  the  inter-relationships  between  the  various 
members  of  a  group  of  plants. 


science  of  Phylogeny,  which  endeavors,  through  a  study  of  fossil 
history,  comparative  anatomy,  and  other  sources,  to  trace  out 
the  complicated  problem  of  ancestry  and  descent  throughout 
the  whole  plant  kingdom.  The  findings  of  this  science  are  of 
great  importance  in  providing  a  basis  for  classification.  While 
there  are  still  differences  of  opinion  as  to  facts,  everyone  is  agreed 
that  the  ideal  to  be  attained  is  a  system  of  classification  which 
is  truly  a  "natural"  one,  or  based  on  descent.  As  our  knowledge 
of  the  evolutionary  history  of  the  plant  kingdom  becomes  more 


258 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


complete,  our  systems  of  classification  will  grow  more  accurate 
and  useful. 

Groups  within  Groups. — Plant  classification  is  far  more  com- 
plex, however,  than  a  mere  segregation  of  individuals  into  a 
series  of  distinct  groups,  for  each  group  is  further  subdivided 
into  smaller  ones  and  each  of  these,  in  turn,  into  others  still 


D  E 

Fig.  140. — Species  belonging  to  the  same  genus.  Six  species  of  maple  (Acer) 
represented  by  leaf  and  fruit.  Each  species  is  readily  distinguishable  from  the 
rest  by  its  own  individual  peculiarities,  but  in  all  the  species  the  fundamental 
characteristics  of  the  maple  genus  are  evident.  A,  Acer  Pennsyhanicum.  B, 
A.  platanoides.  C,  A.  spicatum.  D,  A.  rubrum.  E,  A.  saccharinum.  F,  A. 
saccharum. 


smaller.  This  system  of  "groups  within  groups"  is  familiar 
in  methods  of  classifying  all  sorts  of  objects.  An  army,  for 
example,  has  a  definite  system  of  organization  or  classification, 
whereby  it  is  separated  into  a  series  of  large  groups,  or  divisions. 
Each  division,  in  turn,  is  made  up  of  brigades;  each  brigade,  of 
regiments;  each  regiment,  of  battalions;  each  battalion,  of  com- 
panies; each  company,  of  platoons;  and  each  platoon,  of  squads. 
In  this  way  every  soldier  occupies  a  definite  and  particular  place 


THE  PLANT  KINGDOM  259 

in  the  organization.  In  the  taxonomy  of  the  plant  kingdom  a 
somewhat  similar  series  of  groups  within  groups  has  been 
recognized  and  its  parts  named,  precisely  as  in  a  military  organi- 
zation. The  most  important  of  these  groups  are  the  species, 
genus,  faintly,  order,  class  and  division,  although  additional 
ones  are  often  employed.  Thus  all  the  individuals  which  are 
like  one  another  are  grouped  together  and  constitute  a  species, 
of  which  we  may  take  the  Dog  Rose  as  an  example.  This 
species  is  clearly  very  similar  to  a  large  number  of  other  species, 
the  Prairie  Rose,  the  Swamp  Rose,  the  Sweetbriar  and  many 
others.  All  of  these  species,  which  we  presume  have  descended 
from  the  same  ancestors  originally,  are  grouped  together  as  the 
Rose  genus  (plural,  genera,  Fig.  140),  the  scientific  name  for  which 
is  Rosa;  and  each  of  the  species  also  has  a  scientific  name  of  its  own, 
which  for  the  Dog  Rosa  is  canina.  From  the  general  structure 
of  their  flowers,  fruit  and  other  organs,  we  believe  that  this  Rose 
genus  is  closely  related  to  other  somewhat  similar  genera,  such 
as  the  Cherries  {Prunus),  the  Apples  (Malus),  the  Hawthorns 
(Cra^aeg-Ms),  the  Blackberries  (Ruhus),  the  Strawberries  {Fragaria), 
and  others;  and  we  therefore  group  all  of  these  genera  together 
into  a  still  larger  unit,  the  family,  which  in  this  case  is  the  Rose 
family  or  Rosaceae.  This  is  a  large  family,  containing  about  40 
genera  and  3,000  species.  It  is  evidently  similar  in  many  respects 
to  certain  other  families,  notably  the  Saxifrage  family  (Saxifraga- 
ceae)  and  the  Legume  family  (Leguminosae).  This  group  of 
families,  which  stand  somewhat  by  themselves  and  are  probably 
all  related  to  one  another  by  descent,  we  call  an  order,  in  this 
case  the  Rosales*  The  Rosales  are  one  of  a  large  number  of 
orders  which  constitute  the  great  class  of  Dicotyledoneae  or 
dicotyledonous  plants.  This  is  clearly  distinguished  from 
another  great  class,  the  Monocotyledoneae  or  monocotyledonous 
plants;  and  these  two  classes  comprise  the  subdivision  which  we 
call  the  Angiospermae  or  angiosperms.  The  angiosperms, 
together  with  the  more  ancient  but  now  much  smaller  subdivision 
Gymnospermae,  or  gymnosperms,  make  up  the  division  known 
as  the  Sprrmatophyta  or  seed  plants,  with  which  we  have  already 
become  acquainted  as  one  of  the  four  main  groups  into  which 
the  plant  kingdom  is  divided. 

*  In  generul,  the  scientific  name  of  a  family  has  tlie  ending  -ccac,  tliat  of 
an  order,  -ales. 


260  BOTANY:  PRINCIPLES  AND  PROBLEMS 

These  units  of  classification  which  we  have  ilkistrated  include 
the  most  important  ones,  but  large  groups  are  often  subdivided 
still  further  for  purposes  of  convenience,  so  that  we  meet  with  the 
terms  variety,  tribe,  section,  sub-family  and  others,  each  of  which 
has  its  assigned  place  in  the  system.  Every  one  of  the  thousands 
of  groups  in  the  plant  kingdom  has  its  own  peculiar  and  distinctive 
characteristics  in  which  it  differs  from  every  other  group  of  similar 
grade,  so  that  a  botanist  is  able  to  place  a  newly  discovered 
species  in  just  the  particular  niche  which  it  should  occupy  with 
reference  to  the  plant  kingdom  as  a  whole. 

Nomenclature. — The  technical  names  for  these  various  groups 
are  derived  from  the  Latin  and  Greek  tongues,  and  although 
many  plants  have ' '  common ' '  names  in  the  language  of  the  country 
where  they  grow,  the  advantages  of  technical  or  "scientific" 
names  are  so  great  that  they  are  almost  exclusively  used 
by  botanists. 

In  earlier  days,  before  our  present  system  of  naming  plants 
had  been  introduced,  the  common  way  in  which  a  botanist 
referred  to  a  given  species  was  to  use  a  cumbrous  descriptive 
phrase,  usually  consisting  of  several  Latin  nouns  and  adjectives. 
As  different  men  often  used  different  words,  it  frequently  became 
a  matter  of  doubt  as  to  just  what  plant  they  were  talking  about, 
and  much  confusion  resulted.  It  remained  for  the  genius  of 
the  great  Swedish  naturalist,  Linnaeus,  to  devise  a  method 
which  should  be  simple  and  uniform.  He  invented  the  Binomial 
system  of  nomenclature,  so  called  because  each  species  is  given  two 
names;  first,  the  name  of  genus  of  which  it  is  a  member,  or  its 
generic  name,  and  following  this  a  name  applied  distinctively  to 
the  particular  species  in  question,  or  its  specific  name.  The 
scientific  name  of  the  Dog  Rose  would  thus  be  Rosa  canina. 
This  system  is  very  much  like  that  used  by  us  in  naming  individ- 
uals, where  the  "surname"  is  that  of  a  person's  family  and  the 
"given  name"  is  distinctively  his  own.  In  plants,  this  order  is 
simply  reversed,  the  surname  (generic  name)  coming  first.  The 
binomial  system  was  first  used  extensively  for  plants  in  the 
"Species  Plantarum",  a  great  work  published  by  Linnaeus  in 
1753,  in  which  he  described  all  plant  species  then  known.  This 
book  is  the  foundation  upon  which  our  modern  system  of  plant 
nomenclature  is  based. 

In  order  to  avoid  confusion  and  to  make  perfectly  clear  what 
plant  is  meant,  there  is  placed  after  the  plant  name  the  name 


THE  PLANT  KINGDOM  261 

(or  its  abbreviation)  of  the  botanist  who  first  used  this  name  for 
the  species  in  question.  Thus  the  full  name  of  the  Dog  Rose  is 
Rosa  canina  L.,  which  means  that  this  particular  name  was 
conferred  on  the  plant  by  Linnaeus.  Disputes  still  arise  as  to 
just  what  certain  species  should  be  called,  for  different  botanists 
have  sometimes  given  different  names  to  the  same  plant.  Such 
questions  must  be  settled  by  the  adoption  of  universal  rules 
and  practices,  and  it  is  to  be  hoped  that  through  them  the 
nomenclature  of  plants  may  in  time  become  perfectly  uniform 
throughout  the  world. 

With  this  introduction  to  the  history,  classification,  and  nomen- 
clature of  plants^  we  shall  proceed  in  the  next  four  chapters  to 
describe  briefly  the  main  features  of  the  four  great  divisions  of 
the  vegetable  kingdom. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

639.  Do  you  think  that  the  first  organisms  to  appear  on  the  earth 
were  more  nearly  like  plants  or  like  animals?     Why? 

640.  It  is  generally  agreed  that  the  earliest  plants  lived  in  the  water. 
What  evidence  can  you  think  of  for  this? 

641.  Do  you  think  that  the  earliest  plants  were  motile  or  not?     Why? 

642.  What  are  the  advantages  and  the  disadvantages  of  a  many- 
celled  as  compared  with  a  single-celled  plant? 

643.  What  basis  would  you  use  to  determine  whether  a  group  of  cells 
is  a  colony  or  a  single  plant? 

644.  In  what  way  is  an  animal  individual  a  more  distinct  and  definite 
thing  than  a  plant  individual? 

645.  Why  is  the  increased  size  of  the  plant  body,  beyond  a  certain 
point,  necessarily  followed  by  the  beginning  of  differentiation? 

646.  Why  were  the  sexual  cells  the  first  ones  to  become  differentiated 
from  the  ordinary  body  cells  of  the  plant? 

647.  In  general,  is  there  a  higher  degree  of  differentiation  in  the  body 
of  a  water  plant  or  of  a  land  plant?     Explain. 

648.  State  all  the  resemblances  you  can  find  Iwtweon  a  plant  and  a 
civilized  nation. 

649.  What  oilw.v  advantages  can  you  think  of,  aside  from  increased 
vigor,  which  sexual  reproduction  might  possess  over  asexual? 


262  BOTANY.  PRINCIPLES  AND  PROBLEMS 

650.  In  sexual  reproduction,  what  is  the  advantage  in  having  the  two 
types  of  gametes  (male  and  female)  so  radically  different  from  one 
another? 

651.  Gametes,  particularly  male  gametes,  are  often  motile  when  all 
the  other  cells  of  the  plant  are  not.  How  do  you  think  that  this  has 
come  about? 

652.  What  various  advantages  and  disadvantages  can  you  think  of 
in  a  life-cycle  which  shows  an  alternation  of  generations? 

653.  Why  do  you  think  it  is  that  the  alternation  of  generations,  so 
well  marked  in  lower  plants,  has  practically  disappeared  in  the  highest 
ones? 

654.  Do  you  think  that  plants  or  animals  were  the  first  organisms  to 
migrate  from  the  sea  to  the  land?     Why? 

655.  When  the  first  plants  invaded  the  dry  land,  with  what  kind  of  soil 
did  they  probably  find  it  covered?  What  important  changes  did  the 
presence  of  plant  life  make  in  the  soil? 

656.  Give  a  description  of  the  probable  appearance  of  the  land 
surface  before  the  evolution  of  the  Pteridophytes.  What  regions  on 
earth  today  do  you  think  it  most  closely  resembled? 

657.  What  effect  did  the  evolution  of  the  Pteridophytes  probably  have 
on  the  abundance  of  land  animals?     Why? 

658.  Many  fungi  now  live  entirely  on  land.  Do  you  think  that  they 
were  the  first  land  plants?     Explain. 

659.  What  are  the  advantages  of  the  seed  over  the  spore  as  an  agency 
for  reproduction? 

660.  Why  have  seed  plants  largely  superseded  pteridophytes? 

661.  In  what  way  has  the  evolution  of  seed  plants  probably  changed 
the  characteristics  of  plant-eating  animals? 

662.  What  is  the  practical  use  of  having  a  definite  system  of  classify- 
ing plants  into  species,  genera,  families,  and  other  groups? 

663.  What  organs  are  chiefly  used  as  a  basis  for  the  classification  of 
plants?     Why? 

664.  What  is  an  "artificial"  as  opposed  to  a  "natural"  system  of 
cla.ssification? 

665.  Classify  the  following  objects  into  a  system  of  "groups  within 
groups,"  stating  briefly  the  characteristics  by  which  each  group  may 


THE  PLANT  KINGDOM  263 

bo  distinguished  from  its  coordinate  groups,  and  making  in  this  way  what 
is  commonly  known  as  a  "key"  to  these  objects:  Apple,  oak  log,  pump- 
kin, maple  leaf,  cotton  fiber,  apple  blossom,  potato,  tulij)  bulb,  peanut, 
turnip,  pine  cone,  peach,  spruce  shingle,  strawberry,  automobile  tire, 
squash  seed,  blade  of  grass,  strip  of  birch  bark. 

666.  In  describing  the  plant  kingdom  in  the  later  chapters  of  this 
book,  much  more  space,  relative  to  their  number  of  species,  has  been 
given  many  of  the  lower  groups  than  has  been  given  the  angiosperms. 
Why  is  this  justifiable? 

667.  State  what  advantages  and  disadvantages  the  scientific  name  of  a 
plant  has  as  compared  with  its  "common"  name. 

668.  In  popular  literature  we  often  find  that  when  the  scientific  name 
of  an  animal  or  plant  is  mentioned,  it  has  an  article  in  front  of  it,  as, 
for  example,  "the  Solanum  tuberosum."     Why  is  this  incorrect? 

REFERENCE  PROBLEMS 

108.  What  is  meant  by  the  "life-history  "  of  a  plant  species? 

109.  Who  was  chiefly  responsible  for  the  establishment  of  our  modern 
conception  of  the  Alternation  of  Generations? 

110.  In  general,  how  do  characters  which  distinguish  the  larger  groups 
of  plants,  such  as  orders  and  families,  differ  from  those  which  distinguish 
smaller  ones,  such  as  genera  and  species? 

111.  Why  is  it  that  Latin  and  Greek  are  the  languages  from  which  the 
scientific  names  of  plants  and  animals  have  been  chiefly  derived  ? 

112.  What  is  the  scientific  name  of  the  American  Elm?  of  the  Paper 
Birch?     of  the  Apple?     of  the  Cowslip?     of  the  Blue  Flag? 

113.  Find  the  species,  genus,  family,  order,  class,  subdivision  and  division 
to  which  each  of  the  following  plants  belongs,  and  give  the  correct  scientific 
name  of  each:  The  White  Fine;  the  Red  Oak;  the  Common  Field  Daisy; 
the  Tiger  Lily. 

114.  If  two  botanists  each  give  a  different  name  to  the  same  plant  species, 
what  is  it  that  determines  which  name  shall  be  accepted  as  the  correct  one? 

115.  What  is  meant  by  the  "flora"  of  a  region?  by  the  "moss  flora"  of 
a  region?     by  the  "forest  flora"  of  a  region? 

116.  Give  the  derivation  of  the  following  terms  and  explain  in  what 
way  each  is  appropriate: 

Gamete  Sporophyte  Genus 

Zygote  Gametophyte  Species 


CHAPTER  XIV 
THE  THALLOPHYTA 

The  most  simple  and  primitive  of  the  four  divisions  of  the 
plant  kingdom  are  the  ThallopJnjta  or  thallophytes.  This  is 
a  huge  assemblage  of  species,  about  80,000  in  all,  which  dis- 
play a  wide  varietj^  in  their  structure  and  life  histories.  The 
name  "  thallus-plants "  refers  to  the  character  of  their  vegeta- 
tive body,  which  is  typically  a  thallus,  or  mass  of  tissue  with  little 
differentiation  into  such  diverse  organs  as  we  find  among  the 
higher  plants.  It  is  usually  rather  small,  and  is  often  minute. 
This  simplicity  of  their  vegetative  structures,  together  with 
their  generally  simple  and  primitive  methods  of  reproduction, 
are  the  chief  features  which  distinguish  the  thallophytes  as 
a  whole. 

To  construct  a  truly  natural  classification  for  such  a  hetero- 
geneous group  is  a  very  difficult  task  indeed.  The  division  as  a 
whole  is  usually  separated  into  two  main  series:  The  Algae,  which 
possess  chlorophyll  or  a  similar  substance  and  may  thus  live 
independently,  and  which  include  all  the  seaweeds,  together  with 
the  pond  scums  and  similar  plants  of  fresh  water;  and  the  Fungi, 
which  lack  chlorophyll  and  can  therefore  exist  only  as  saprophytes 
or  parasites,  and  to  which  belong  the  multitude  of  bacteria,  molds, 
mildews,  blights,  rusts,  toadstools,  and  mushrooms.  The  fungi 
have  evidently  arisen  from  several  different  groups  of  algae,  so 
that  the  two  series  parallel  one  another  somewhat  in  their  various 
characteristics.  It  is  more  convenient  to  treat  each  separately'' 
however,  incidentally  pointing  out  such  relationships  as  seem 
clear  between  various  groups  in  the  two  series. 

THE  ALGAE 

The  algae  are  commonly  divided  into  four  classes,  the  Blue- 
green  Algae,  the  Green  Algae,  the  Brown  Algae,  and  the  Red 
Algae.  The  differences  in  color  which  have  given  rise  to  these 
names  are  incidental  and  are  accompanied  by  more  deeply 
seated  distinctions. 

264 


THE  rilALLOPHYTA  265 

Cyanophyceae  or  Blue-green  Algae. — These  are  the  simplest 
and  lowliest  of  all  green  plants.  The  body  consists  of  a  single 
cell,  but  in  most  species  the  cells  tend  to  hold  together  in  colonies. 
The  cell  itself  is  very  simple,  lacking  the  nucleus,  sap-cavity,  and 
chloroplastids  so  characteristic  of  other  green  plants.  Its  cyto- 
plasm may  be  perfectly  homogeneous,  with  the  pigment  evenly 
dispersed,  or  a  colored  outer  zone  and  a  colorless  inner  one  may  be 
roughly  distinguishable.  The  latter  perhaps  represents  a  nucleus. 
The  pigment,  which  seems  to  be  dissolved  directly  in  the  cyto- 
plasm and  never  confined  to  definite  plastids,  is  usually  (though 
not  always)  blue-green  in  color  and  is  probably  a  combination  of 
chlorophyll  with  a  blue  pigment,  phycocyanin.  Both  of  these 
may  be  concerned  with  photosynthesis  but  this  is  as  yet  uncertain, 
for  our  knowledge  of  the  photosynthetic  process  in  the  blue- 
green  algae  is  far  less  complete  than  it  is  for  higher  plants.  The 
cell-wall  is  typically  thick  and  mucilaginous,  and  in  many  species 
a  group  of  cells  becomes  embedded  in  the  gelatinous  mass  derived 
from  their  walls  so  that  a  large,  jelly-like  colony  results.  On  the 
occurrence  of  unfavorable  conditions  for  growth,  heavy-walled 
"resting  cells"  may  be  produced.  Cell  division  is  very  simple 
and  shows  none  of  the  elaborate  phenomena  of  mitosis,  the  cell 
merely  becoming  constricted  by  the  growth  of  a  new  wall  until 
complete  separation  into  two  cells  takes  place.  Little  differentia- 
tion is  evident,  although  peculiar  large,  dead  cells  frequently 
appear  in  certain  species.  Reproduction  consists  merely  in  cell 
division  or  "fission,"  and  no  instances  of  sexuality  have  ever 
been  observed  in  the  class.  In  this  respect  the  blue-greens  differ 
from  other  algae  but  resemble  bacteria,  and  these  two  groups 
have  therefore  sometimes  been  placed  together  as  a  separate 
division  of  the  plant  kingdom,  the  Schizophyta  or  Fission  Plants, 
divided  into  the  classes  Schizophyceae  (Cyanophyceae)  and 
Schizomycetes  (Bacteria). 

The  blue-green  algae  live  in  both  salt  and  fresh  water  and  are 
able  to  grow  at  higher  temperatures  than  can  any  other  plants, 
often  thriving  in  the  water  of  hot  springs  at  temperatures  up  to 
60°C.  Most  species  prefer  water  which  is  dirty  and  full  of 
organic  matter,  and  some  may  even  be  found  on  damp  soil, 
rocks,  and  other  places  which  are  exposed  to  the  air. 

One  of  the  simplest  examples  is  Gloeocapsa  (Fig,  141,  B),  a 
minute,  single-celled  alga  with  a  very  gelatinous  wall.  As  an 
individual  divides,  the  resulting  cells  of  the  first  generation,  and 


266 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


sometimes  those  of  the  third  and  fourth,  are  held  together, 
embedded  in  their  swollen  cell-walls.  In  Nostoc  (Figs.  141,  C 
and  D)  the  individuals  are  joined  loosely  into  filaments  which 
somewhat  resemble  strings  of  beads,  and  these  may  also  be 
embedded  in  a  jelly-like  substance,  the  whole  colony  often  reaching 
a  diameter  of  one  centimeter  and  containing  hundreds  of  filaments. 


Fig.  141. — Cyanophyceae  of  various  sorts.  ^4,  Oscillatoria,  X  340.  B, 
Gloeocapsa.  X  450.  C,  iVosioc  colony,  natural  size.  Z),  filament  of  iVostoc,  with 
a  heterocyst  in  the  middle  of  it.  X  460.  E,  colony  of  Rivularia.  X  M-  -P. 
filament  of  Rivularia,  with  heterocyst  at  base.  X  225.  {E  and  F  after  Engler 
and  Prantl) . 

Here  and  there  along  the  filament  are  frequently  found  large, 
empty  cells  called  heterocysts.  Their  function  is  not  definitely 
known  but  they  may  be  concerned  in  breaking  up  the  filament 
into  short  segments  or  hormogonia.  In  Oscillatoria  (Fig.  141,  A) 
the  cells  are  pressed  flatly  against  one  another  and  the  gelatinous 
wall  is  so  poorly  developed  that  the  filaments  are  free  in  the  water, 
where  they  sway  or  revolve  slowly.  It  is  hard  to  know  whether 
to  regard  the  filament  of  some  of  these  blue-green  algae  as  a 
colony  of  distinct  individuals  or  as  a  single,  many-celled  plant. 
Chlorophyceae  or  Green  Algae. — This  class  is  by  far  the  largest 
of  the  four  groups  of  algae  and  its  members  are  very  diverse. 
They  contain  chlorophyll  but  no  other  pigment,  and  the  bright 


THE  TITALLOPIIYTA  2G7 

green  color  which  the  plant  body  thus  displays  has  given  the  class 
its  name.  It  is  well  represented  in  both  fresh  and  salt  water  and 
a  few  species  thrive  in  damp  situations  on  land.  The  cell  is 
much  more  highly  differentiated  here  than  among  the  blue-green 
algae,  possessing  a  nucleus,  one  or  more  chloroplasts  (often 
called  chromatophores)  and  usually  a  sap-cavity,  thus  resembling 
in  its  essential  details  the  cells  of  the  higher  plants.  Pyrenoids, 
or  centers  of  starch  formation,  are  prominent  in  the  chloroplasts. 
The  plant  body  may  consist  of  a  single  cell,  a  filament,  or  a  plate 
of  cells.  Most  species  (though  not  all)  produce  zoospores,  motile 
reproductive  cells  which  swim  about  by  the  aid  of  one  or  more 
lashes  or  cilia  and  which  grow  directly  into  new  plants.  These 
are  developed  in  modified  cells  or  sporangia.  Various  types 
of  sexual  reproduction  are  also  found  in  this  class,  ranging 
from  instances  where  the  gametes  are  entirely  similar  to  those 
where  they  have  become  markedly  distinguishable  as  sperms 
and  eggs.  Because  of  all  this  structural  diversity  and  of  the  fact 
that  they  are  thought  to  be  near  the  main  line  of  ascent  from 
lower  algae  to  bryophytes,  the  Chlorophyceae  have  received 
intensive  study,  particularly  with  regard  to  the  development 
of  the  multicellular  individual  and  the  evolution  of  sex. 

To  classify  this  great  class  thoroughly  it  is  necessary  to  dis- 
tinguish within  it  a  large  number  of  orders,  but  for  the  purposes 
of  our  brief  survey  we  can  conveniently  group  these  into  five: 
The  Protococcales,   the   Confervales,   the  _ 

Conjugales,    the    Siphonales,     and    the 
Charales. 

1.  Protococcales  or  One-celled  Green 
Algae. — These  are  chiefly  microscopic 
plants.  The  individuals  are  single-celled 
and  they  may  be  completely  separate 
or  loosely  joined  into  colonies,  and  are 
either  motile  or  non-motile.  Fig.  142. — PUurococcus. 

Plei,rococcus  (Fig.  142),  which  forms  ^^Vol' c^enl '"x  ISi'" 
the  green   stain   found   on   damp  bark, 

rocks,  and  similar  places,  is  perhaps  the  most  common  type.  It 
consists  of  a  single  cell  containing  one  large  chloroplastid  and 
reproduces  only  by  cell  division.  The  daughter  cells  may  some- 
times cohere  for  a  time  in  small  groups.  This  is  one  of  the 
algae  which  is  commonly  associated  with  various  fungi  to  form 
the  peculiar  group  of  lichens. 


268 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Chlamijdomonas  (Fig.  143)  is  a  motile  type,  its  cells  containing  a 
single  chloroplastid,  a  red  pigment  spot,  some  contractile  vacuoles, 
and  two  cilia.*  The  cell  may  lose  its  cilia  and  divide  into  several 
zoospores,  each  of  which  is  capable  of  producing  a  new  plant. 
Toward  the  end  of  the  growing  season,  however,  smaller  motile 


.1 

Fig.  143. — Chlamydomonas.  A,  individual  plant,  showing  the  large  chloro- 
plast,  the  pyrenoid,  the  nucleus  and  the  two  cilia.  B,  union  of  two  gametes. 
In  most  instances  these  gametes  are  equal  in  size,  but  in  the  case  figured  they  are 
distinctly  different.     (B  after  Goroschankin) . 

cells  are  formed  which  swim  about  and  unite  in  pairs.  These 
are  the  gametes,  and  in  this  case  their  evolution  from  ordinary 
zoospores  seems  to  be  very  clear.  The  cell  formed  by  their 
union  is  known  as  a  zygospore,  a  term  applied  to  all  cells  produced 
by  the  fusion  of  similar  gametes.  Such  a  form  of  sexual  reproduc- 
tion is  known  as  isogamy. 

*  This  plant  is  very  similar  to  members  of  the  interesting  group  of  Flagel- 
lates, sometimes  classed  with  animals  and  sometimes  with  plants,  and  which 
are  evidently  intermediate  between  the  two. 


THE  THALLOPHYTA 


269 


There  are  a  number  of  other  algae  similar  to  Chlamydomonas 
except  that  they  arc  united  into  loose  colonies  of  ciliated  indi- 
viduals, and  the  group  culminates  in  the  large,  hollow,  spherical 
colonies  of   Volvox   (Fig,  144)  which  often  consist  of  thousands 


Fig.  144. —  Volvox.  Globular  colony  of  bi- 
ciliate  individuals,  with  three  daughter- 
colonies  developing  in  its  interior. 


^^-B 


—  Volvox.  A,  group 
of  sperms  developed  within  a 
single  cell.  B,  sperms.  C, 
fertilized  egg  or  oospore.  {After 
Cohn) . 


of  cells.  In  these  higher  forms  the  sperms  are  markedly  different 
from  the  eggs,  and  the  cell  formed  by  their  union  is  termed  (as 
in  all  such  cases)  a  fertilized  egg  or  oospore  (Fig,  145),     Sexual 


^.M^. 


nw 


Tfi  '■ 


Fig.  146. — Pediastrum.  A  group 
of  sixteen  cells  united  into  a  plate- 
like colony.      X  260. 


Fig.  147. — Hydrodictyon.  Portion  of 
a  net-like  colony,  individual  cells  forming 
the  meshes  of  the  net.      X  310. 


reproduction  of  this  type  is  known  as  heterogamy.  It  should  be 
noted  that  both  zygospores  and  oospores  are  typically  thick- 
walled  cells  which  are  capable  of  resisting  such  unfavorable  condi- 
tions as  drought  and  low  temperature,  and  of  germinating  to 


270  BOTANY:  PRINCIPLES  AND  PROBLEMS 

produce  new  plants  whenever  a  suitable  environment  again 
appears. 

In  another  group  of  Protococcales  the  body  cells  are  non- 
motile,  independent  movement  being  limited  here  to  the  zoos- 
pores and  the  gametes.  The  individuals  are  grouped  in  colonies 
which  are  simple,  few-celled  plates  in  Pediastriim  (Fig.  146)  but 
form  a  complex  network  in  the  water-net,  Hydrodidyon  (Fig.  147). 
The  zoospores  do  not  escape  from  the  mother  plant  and  swim 
about,  as  is  usually  the  case,  but  the  group  of  zoospores  formed 
within  a  single  mother-cell  displays  the  remarkable  habit  of 
uniting,  while  still  within  this  cell,  to  form  a  minute  colony  of 
non-motile  cells  which  is  finally  liberated  and  grows  into  a 
mature  colony. 

The  Protococcales  are  of  especial  interest  because  of  the  light 
which  they  throw  upon  the  development  of  the  multicellular 
individual  and  the  differentiation  of  the  sexes. 

2.  Confervales  or  Conferva-like  Algae. — These  include  most 
of  the  common  thread-like  and  membranous  green  algae  of  salt 
and  fresh  water.  They  all  reproduce  asexually  by  zoospores  and 
also  exhibit  various  types  of  sexual  reproduction,  simple  in  the 
lower  forms  but  relatively  complex  in  the  higher  ones.  The  order 
is  large  and  varied  and  contains  many  species  which  are  but 
distantly  related  to  one  another.  Three  typical  genera  will  give 
us  an  idea  of  the  group. 

Ulothrix  (Fig.  148)  is  a  common  thread-like  or  filamentous  alga, 
its  short  cells  each  containing  a  single  nucleus  and  one  large 
chloroplast.  The  contents  of  any  cell  may  become  divided 
into  a  group  of  zoospores  which  escape  and  may  each  form  a  new 
plant ;  and  smaller  binucleate  motile  cells,  produced  in  the  same 
manner  as  the  zoospores  and  often  indistinguishable  from  them, 
act  as  gametes  and  conjugate  in  pairs  to  produce  zygospores. 
Ulothrix  is  often  cited  as  another  good  example  of  the  origin  of 
sexual  reproduction. 

Oedogonium  (Fig.  149)  is  also  a  common  genus  and  its  filament 
is  anchored  by  a  modified  basal  cell,  the  holdfast.  In  certain  cells 
the  contents  may  round  up  and  produce  a  single  large  zoospore 
with  a  circle  of  cilia  near  one  end,  and  this  soon  settles  down, 
develops  a  holdfast,  and  grows  into  a  new  filament.  Other  cells 
also  become  much  enlarged,  each  producing  a  single  rounded, 
non-ciliated  cell,  well  supplied  with  chloroplastids  and  food 
material.     This  is  the  female  gamete  or  egg,  and  the  cell  which 


THE  THALLOPHYTA 


271 


produces  it  (like  all  egg-producing  structures  in  these  lower 
plants)  is  termed  an  odgonium.  In  other  cells  the  contents  divide 
into  two  small,  motile  male  gametes  or  sperms,  and  each  of  these 
mother-cells  (like  all  structures  which  produce  male  gametes) 
is  known  as  an  antheridium.     One  of  the  sperms  enters  an  egg 


Fig.  148.  Fig.   149. 

Fig.  148  — Ulothrix.  A,  young  filament  with  rhizoid  cell  (r)  by  which  it  is 
attached.  B,  portion  of  filament  with  escaping  zoospores.  C,  single  zoospore. 
D,  formation  and  escape  of  gametes.  E,  gametes.  F  and  G,  stage  in  the 
conjugation  of  two  gametes.  //,  zygote  or  zygospore.  I,  zygote  beginning  to 
germinate.  J,  group  of  zoospores  produced  by  a  zygote.  (From  Strasburger, 
after  Dodel-Port). 

Fig.  149. — Oedogonium  nodulosum.  A,  filament  with  antheridium  (a),  each 
cell  of  which  produces  two  sperms;  and  oogonium  (o),  containing  one  large  egg. 
B,  filament  with  a  thick-walledo  ospore  {os)  which  has  developed  from  a  fertilized 
egg.     C,  basal  cell  of  a  filament,  showing  holdfast.     All  X  300. 

and  fertilizes  it,  and  the  oospore  thus  formed  germinates  into  a 
group  of  zoospores. 

Coleochaete  (Fig.  150)  is  a  fresh-water  alga  the  vegetative  body 
of  which  consists  of  a  flat  plate  or  cushion  of  radiating  filaments. 
Its  cells  may  produce  single,  large,  biciliate  zoospores.     Antheri- 


272 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


dia  and  oogonia  are  formed  much  as  in  Oedogonium,  and  a 
thick-walled  oospore  is  produced.  Following  this,  however,  the 
adjacent  cells  give  rise  to  branches  which  grow  up  and  surround 
the  oospore,  forming  a  distinct  spore  case  or  fruiting  body.  The 
oospore  germinates  into  a  group  of  cells  each  of  which  ultimately 
forms  a  zoospore.  This  reproductive  cycle  foreshadows  the 
"alternation  of  generations"  of  the  higher  plants.     In  its  struc- 


r^  m  L 


Fig.  150. — Coleochaete.  A,  portion  of  fertile  thallus.  og,  oogonium,  contain- 
ing a  single  large  egg.  an,  antheridium.  s,  sperm,  o,  oospore  or  fertilized  egg, 
which  is  beginning  to  be  surrounded  by  filaments  growing  up  from  below.  B, 
mature  fructification,  the  oospore  surrounded  by  an  envelope  of  cells.  C,  the 
contents  of  the  fructification  dividing  up  into  zoospores.  D,  zoospores.  (From 
Goebel,  after  Pringsheim) . 

ture  and  life  history,  Coleochaete  is  the  most  specialized  of  the 
green  algae  and  is  believed  by  many  botanists  to  approach  the 
lowest  bryophytes. 

3.  Siphonales  or  Tubular  Algae. — These  are  distinguished 
from  all  other  algae  by  the  fact  that  the  whole  plant  body, 
whether  it  be  a  simple  filament  or  a  well-differentiated  thallus,  is 
essentially  a  single  cell.  The  cross  walls  which  divide  other 
algae  and  all  ordinary  plants  into  small  cells  are  absent,  and 
the  mass  of  cytoplasm  with  its  thousands  of  nuclei  is  therefore 
able  to  circulate  freely  throughout  the  whole  plant.  Such  a 
multinucleate  cell,  of  which  these  plants  are  extreme  examples,  is 
known  as  a  coenocyte. 

The  Siphonales  are  chiefly  marine  forms,  especially  abundant 
in  the  warmer  seas.     They  usually  produce  zoospores  and  in 


THE  THALLOPHYTA 


273 


cases  where  sexuality  has  been  proven  are  always  isogamous 
except  in  the  genus  Vauchcria,  which  is  such  a  familiar  and  dis- 
tinctive type  that  we  shall  describe  it  more  fully.  This  alga 
forms  the  common  "green  felt"  so  often  found  on  damp  soil  or 
in  muddy  pools,  and  consists  of  a  tangled  mass  of  coarse,  branch- 


.  Fig.  151. — Vaucheria.  Asexual  reproduction.  The  tip  of  a  filament  is  cut 
off  by  a  wall  and  its  contents  becomes  a  large  zoospore,  with  many  nuclei  and 
many  groups  of  paired  cilia.  The  zoospore  breaks  through  the  wall  and  escapes. 
{After  Gotz.) 

ing,  tubular  filaments.  Large  zoospores  are  produced,  each  of 
which  is  merely  the  contents  of  the  tip  of  a  filament  which  has 
been  cut  off  by  a  wall  and  has  escaped  (Fig.  151).  The  sexual 
organs  are  not  simply  modified  vegetative  cells,  as  in  the  plants 


Fig.  152. — Vaucheria.  Sexual  reproduction.  Oogonia,  o,  each  contain  a 
single  egg.  Antheridia,  a,  have  discharged  all  their  sperms  and  are  empty.  A, 
V.  terrestris.     B,  V.  sessilis  X  130. 


previously  studied,  but  arc  specialized  for  gamete  production 
(Fig.  152).  A  cell  partitioned  off  by  a  wall  from  the  main  fila- 
ment or  from  a  small  lateral  branch  becomes  the  oogonium,  within 
which  a  single,  large  egg  is  formed.     From  the  tip  of  another 


274 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


small  branch  near  by  is  cut  off  a  cell  which  develops  into  an 
antheridium.  One  of  the  sperms  here  developed  enters  the 
oogonium  and  fertilizes  the  egg,  producing  a  heavy-walled  oospore. 
4.  Conjugales,  the  Pond  Scums,  Desmids,  and  Diatoms. — The 
plant  body  of  these  algae  is  a  single  cell  or  an  unbranched  fila- 
ment.    The  species  are  all  confined  to  fresh  water  and  are  dis- 


FiG.  153. — Spirogyra.  Two  adjacent  filaments,  showing  stages  in  sexual 
reproduction.  Cell  a  shows  the  normal,  resting  condition,  with  the  spirally 
placed,  band-like  chromatophore,  in  which  numerous  circular  pyrenoids  can  be 
seen.  Cells  b  and  c  are  sending  out  conjugating  tubes  to  each  other.  The  con- 
tents of  cells  d  and  e  have  contracted  somewhat  and  the  contents  of  e  is  passing 
over  through  the  conjugating  tube  and  uniting  with  d.  At /is  shown  the  mature 
zygospore  which  has  arisen  from  the  union  of  two  cells.       X  150. 


tinguished  from  other  Chlorophyceae  by  the  absence  of  zoospores 
or  other  means  of  asexual  reproduction,  the  absence  of  motile 
cells  of  any  sort,  the  occurrence  of  large  and  conspicuous  chloro- 
plastids,  and  the  characteristic  manner  in  which  sexual  reproduc- 
tion is  brought  about. 


THE  TIIALLOPHYTA 


275 


The  Pond  Scums,  of  which  Spirogyra  (Fig.  153)  is  the  common 
example,  are  all  filamentous  algae.  In  this  genus  the  chloroplast 
is  a  broad,  strap-shaped  structure  running  spirally  around  the  cell 
and  on  it  appear  a  series  of  small,  rounded  areas,  the  pyrenoids. 
The  nucleus  is  suspended  in  the  middle  of  the  sap  cavity  by 
threads  of  cytoplasm  extending  to  the  walls.  In  sexual  reproduc- 
tion, adjacent  cells  of  two  filaments  which  are  lying  side  by  side 


m 


^M 


Fig.   154. — Desmids  of  various  tyj 


send  out  projections  or  "conjugating  tubes"  toward  one  another. 
The  tips  of  these  touch,  the  wall  between  them  breaks  down,  and 
through  the  channel  thus  formed  the  whole  protoplasmic  con- 
tents of  one  cell  enters  the  other  and  the  living  portions  of  the 
two  cells  fuse  into  a  thick-walled  zygospore.  Occasionally  the 
two  cells  conjugate  in  the  tube  itself,  and  sometimes  two  adjacent 
cells  of  the  same  filament  may  unite. 

The  Desmids  (Fig.  154)  are  unicellular  plants  of  the  utmost 
variety  and  beauty  of  form.  The  cell  is  composed  of  two  per- 
fectly symmetrical  halves  separated  by  a  zone,  the  istJmius,  which 
is  often  constricted  and  under  which  lies  the  nucleus.     Aside 


276 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


from  ordinary  cell  division,  reproduction  is  effected  by  conjuga- 
tion between  two  protoplasts,  each  of  which  has  escaped  from  its 
wall. 

The  Diatoms  (Fig.  155),  a  large  group  of  unicellular  plants 
of  somewhat  uncertain  relationship,  can  perhaps  best  be  con- 
sidered here.  They  are  represented  by  thousands  of  species  in 
salt  and  fresh  water  and  are  the  most  abundant  of  the  minute 
plants  which  make  up  that  multitude  of  freely  swimming  or  float- 


FiG.   155. — Diatoms  of  various  types. 


ing  algae  in  the  open  sea  and  in  fresh  water  which  we  know  as  the 
plankton.  Many  species  are  also  sessile  and  may  be  united  into 
filaments  or  other  colonies.  The  wall  is  hard  and  flinty,  being 
heavily  filled  with  silica,  and  consists  of  two  halves  or  valves  one 
of  which  fits  over  the  other  like  a  box  cover.  These  are  highly 
diversified  in  shape  and  are  frequently  ornamented  with  minute 
and  intricate  markings  which  have  long  been  the  delight  of  micro- 
scopists.  The  color  of  most  diatoms  is  brown  owing  to  the  pres- 
ence of  a  pigment,  diatomin,  in  addition  to  chlorophyll. 


THE  TIIALLOPHYTA 


277 


5.  Chorales  or  Stoneworts  (Fig.  156). — This  remarkable  group 
of  plants  stands  apart  from  all  others  and  we  have  no  certain 
knowledge  as  to  its  relationships.  The  vegetative  body  consists 
of  long,  jointed  stems,  and  at  the  joints,  or  nodes,  arise  whorls  of 


Fig.  156. — Chara.  A,  a  main  branch,  showing  the  circles  of  small,  lateral 
branches,  bearing  the  sexual  organs,  arising  at  the  joints  of  the  main  branch. 
B,  part  of  a  lateral  branch  showing  the  sexual  organs;  the  oogonium  above  and 
the  antheridium  below.  C,  the  sexual  organs  and  branch  in  section,  oogonium 
(with  one  large  egg)  at  right  and  antheridium  at  left. 


short  branches.  No  asexual  spores  are  produced,  but  along  the 
branches  are  borne  antheridia  and  oogonia,  far  more  compli- 
cated than  among  any  other  thallophj^tcs.  The  antheridium  is 
spherical  and  its  wall  is  composed  of  eight  somewhat  triangular 
cells  from  the  inner  surface  of  which  arise  a  large  number  of  many- 


278  BOTANY:  PRINCIPLES  AND  PROBLEMS 

celled,  sperm-producing  filaments.  The  oogonium  is  covered 
with  a  wall  or  envelope  of  spirally  wound  cells  growing  up  from 
the  tissue  below  it,  and  produces  a  single  large  egg.  After  fer- 
tilization the  envelope  hardens,  forming  a  nut-like  spore  case 
around  the  oospore.  The  absence  of  anything  suggesting  an 
alternation  of  generations  indicates  that  these  plants  should  be 
placed  among  the  thallophytes,  but  they  are  clearly  distinct 
from  any  other  members  of  the  division. 

Phaeophyceae  or  Brown  Algae. — These  plants  may  be  dis- 
tinguished from  other  algae  by  their  characteristic  brown  color, 
due  to  one  or  more  brown  pigments  associated  with  chlorophyll, 
and  by  certain  structural  characters.  The  Phaeophyceae  are 
the  largest  and  rankest  of  all  algae  and  display  the  highest  degree 
of  bodily  differentiation.  They  are  found  almost  exclusively  in 
salt  water  and  are  best  developed  in  the  cooler  seas.  Thriving 
most  commonly  in  shallow  water  and  the  zone  between  tide 
marks,  they  are  subjected  to  the  buffeting  of  the  waves  and  may 
be  exposed  to  the  air  for  several  hours  a  day.  These  plants 
probably  represent  an  entirely  independent  line  of  evolution 
from  the  green  algae  and  seem  to  have  led  to  no  higher  types. 
Two  orders  are  recognized  among  them,  the  Phaeosporales  and 
the  Fucales. 

1.  Phaeosporales  or  Kelps  and  Their  Allies. — In  this  order 
occur  the  kelps  (Laminaria)  common  in  the  north  Atlantic  and 
elsewhere,  together  with  many  other  large  algae  such  as  the  giant 
kelp  (Macrocystis) ,  the  sea  otter's  cabbage  (Nereocystis),  the 
sea  palm  (Postelsia),  and  others  smaller  in  size.  They  are  all 
isogamous  and  in  most  cases  produce  zoospores. 

Ectocarpus  (Fig.  157),  one  of  the  best  known  genera,  is  a 
rather  small,  filamentous  plant.  As  in  the  algae  previously 
studied,  the  zoospores  are  here  produced  in  sporangia  which 
are  modified  single  cells.  The  gametes,  however,  are  developed 
in  large  multicellular  structures  (plurilocular  sporangia)  which 
begin  to  show  a  resemblance  to  the  highly  developed  sexual 
organs  characteristic  of  the  bryophytes.  Each  of  the  many 
small  cells  into  which  the  contents  of  this  structure  is  divided 
forms  one  or  two  gametes  which  fuse  in  pairs  to  produce  zygo- 
spores. In,stances  have  been  observed  in  which  these  gametes 
germinate  directly  into  a  new  plant,  and  thus  function  essentially 
like  zoospores,  which  they  also  resemble  structurally.  Gametes 
of  different  size  sometimes  unite,  thus  indicating  the  beginning  of 


THE  TIIALLOPIIYTA 


279 


a  heterogamous  condition.     In  genera  like  Edocarpus  we  are 
evidently  near  the  beginning  of  sexuality  in  the  brown  algae. 

The  larger  forms  or  kelps  may  become  huge  plants,  the  giant 
kelp  sometimes  attaining  a  length  of  from  two  hundred  to  three 
hundred  meters.  They  are  attached  to  the  rocks  by  massive 
holdfasts.     The    stout    "stems"   or  stipes   support   broad   and 


Fig.    157. — Edocarpus.      A,     filament     with      multioellular    or     plurilocidar 
sporangia,  in  which  gametes  are  produced.     A  young  and  growing  sporangium  is 

gamete;  at  left,  the  begi 


n  gametes  are  produced.     A  young  ana  growing  sporangium  is 
filament  with   a   single-celled  sporangium,  in  which  zoospores 


shown  above,  a,  tilament  witn  a  singie-cened  sporangium,  ir 
are  produced.  C,  various  stages  in  the  union  of  two  gametes. 
„„mete;  at  left,  the  beginning  of  union;  at  right,  the  resulting  zy; 
(C  after  Oltmanns) . 


Above,  a  single 
gospore.      X  375. 


frequently  much-divided  blades  and  often  tlisplay  a  certain 
degree  of  structural  differentiation.  The  abundant  gametes 
were  long  thought  to  be  zoospores,  but  their  true  nature  as 
sexual  cells  is  now  known  and  it  may  be  that  zoospores  are  rather 
rare  in  this  group. 

2.  Fucales  or  Rockweeds. — These  plants  differ  from  the  kelps 
in  producing  no  zoospores  and  in  displaying  a  heterogamous 
type  of  sexual  reproduction. 


280  BOTANY:  PRINCIPLES  AND  PROBLEMS 

Fucus,  the  best  known  genus,  is  exceedingly  abundant  on 
rocks  between  tide  marks  in  the  temperate  regions.  The  vegeta- 
tive body  of  this  alga  is  a  flat,  repeatedly  forking  thallus  well 
provided  with  air  bladders  (Fig.  158).     The  swollen  tips  of  certain 


Fig.  158. — Fucus  vesiculosus.  Portion  of  thallus  showing  air-bladders, 
(a)  and  receptacles  (r).  On  the  surface  of  the  latter  are  the  many  small  openings 
of  the  conceptaeles. 

branches  are  known  as  receptacles  and  bear  the  sexual  organs. 
Scattered  over  these  tips  and  just  below  the  surface  are  many 
small  chambers  or  conceptaeles  each  opening  to  the  outside  by 
a  pore.     Lining  the  wall  of  these  chambers  are  masses  of  branch- 


THE  TIIALLOPHYTA 


281 


ing  filaments  among  which  arc  placed  the  antheridia  and  oogonia 
(Fig.  159).  The  antheridia  arc  small  cells  arising  on  branches  of 
the  filaments  and  producing  swarms  of  biciliate  sperms.  The 
oogonia  are  larger  cells,  the  contents  of  each  dividing  into  eight 


Fig.  159. — Fucus.  A,  female  conceptacle,  with  oogonia  and  sterile  filaments. 
B,  single  oogonium  at  maturity,  containing  eight  eggs.  C,  group  of  antheridia 
from  a  male  conceptacle.  D,  sperms.  E,  egg  after  discharge  into  the  water, 
surrounded  by  sperms.  F,  young  plant  arising  from  an  oospore.  (5,  C,  D,  E 
and  F  after  Thurel). 


eggs.  In  some  species  both  sex  organs  are  produced  in  the  same 
conceptacle,  in  others  they  occur  in  different  conceptacles  on  the 
same  plant,  and  in  still  others  the  whole  plant  is  entirely  male  or 
entirely  female.  Both  eggs  and  sperms  are  discharged  from  the 
mouth  of  the  conceptacle  and  fertilization  takes  place  in  the  open 


282 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


water   outside.     The   oospore   settles   to   the   bottom,    attaches 
itself,  and  develops  directly  into  a  new  plant. 

Rhodophyceae  or  Red  Algae. — These  are  a  very  varied 
group,  rich  in  species,  almost  exclusively  marine,  and  reaching 
their  best  development  in  the  warmer  seas.  Most  of  them  grow 
entirely  submersed,  below  tide  marks,   and  are  therefore  not 


Fig.  160. — Nemalion.  A,  procarp  at  end  of  a  filament,  c,  carpogonium  and 
t,  trichogyne,  to  which  several  sperms  are  attached.  A  male  nucleus  has  entered 
the  carpogonium.  B,  cystocarp,  which  has  arisen  from  the  fertilized  carpogon- 
ium. Carpospores  are  developing  at  the  ends  of  short  filaments.  C,  antheridia, 
groups  of  small  cells  each  of  which  produces  a  sperm.      X  60. 


particularly  conspicuous  or  familiar.  The  vegetative  body  tends 
to  be  delicate,  filamentous,  and  much-branched,  in  contrast  to 
the  bulky  thallus  of  the  brown  algae.  The  Rhodophyceae  are 
distinguished  from  all  other  algae  by  their  characteristic  reddish 
color  (due  to  the  pigment  phycoerythrin  which  is  present  with 
chlorophyll);  by  the  complete  absence  of  motile  cells  of  any  kind, 
and  by  a  highly  specialized  type  of  sexual  reproduction.  They 
do  not  include  primitive  types  but  seem  to  have  arisen  from  a 


THE  THALLOPHYTA 


283 


point  already  well  up  in  the  scale  of  algal  evolution.  The  higher 
members  of  the  class  display  an  unquestionable  alternation  of 
generations.  Two  typical  genera  will  illustrate  the  complexities 
of  reproduction  in  this  group. 

Nemalion  (Fig.  160)  is  a  common  form  with  a  branching, 
filamentous  habit  and  represents  the  simplest  of  the  red 
algae.     The    numerous    small    antheridia    profluce    non-motile, 


Fig.  161. — Polysiphonia  fihrillosa.  A,  Portion  of  a  tetrasporic  plant  showing 
three  groups  of  tetraspores.  These  spores  occur  in  tetrads  or  groups  of  four. 
X  90.  B,  portion  of  a  male  plant.  Above,  an  immature  antheridium;  below,  a 
mature  one,  producing  large  numbers  of  sperms  on  its  surface.  X  215.  C, 
portion  of  a  female  plant  showing  a  ripe  cystocarp,  its  wall  enclosing  a  group 
of  carpospores.       X  45. 

binucleate  sperms.  The  female  sexual  organ,  which  is  here 
called  the  procarp,  consists  of  two  cells — a  basal  carpogonium, 
containing  the  single  female  cell  and  homologous  with  the  oogo- 
nium of  other  algae;  and  a  terminal  cell,  the  trichogtjne,  which  is 
drawn  out  into  a  hairlike  tip.  The  sperm  is  carried  bj^  water 
currents  to  the  trichogyne,  down  which  its  contents  passes  into 
the  carpogonium.  The  fertilized  carpogonium  does  not  develop 
a  new  plant  directly,  but  produces  a  group  of  short  filaments  on 
the  end  of  each  of  which  is  borne  a  non-motile  carpospore.  This 
whole  structure  of  spores,  filaments,  and  carpogonium  is  known  as 
the  cystocarp.  A  carpospore  germinates  directly  into  a  new 
plant. 

Polysiphonia  (Fig.  161),  another  common  member  of  tli(>  class, 
displays  a  much  more  complex  sexual  history  and  is  tyj^ical  of 


284  BOTANY:  PRINCIPLES  AND  PROBLEMS 

the  majority  of  the  red  algae.  Three  types  of  plants,  similar  in 
vegetative  structure  but  differing  in  their  reproductive  organs, 
may  be  distinguished:  Male  plants,  producing  antheridia  only; 
female  plants,  producing  procarps  only;  and  sexless  tetrasporic 
plants,  producing  sporangia  in  each  of  which  are  four  asexual 
tetraspores.  The  procarp  is  somewhat  more  complex  than  in 
Nemalion  since  it  contains  a  group  of  auxiliary  cells.  Fertili- 
zation, however,  occurs  in  much  the  same  way,  a  sperm  coming  in 
contact  with  the  trichogyne,  entering  the  carpogonium,  and 
fusing  with  the  female  nucleus.  Some  rather  complicated 
fusions  now  take  place  between  the  fertilized  carpogonium  and  the 
auxiHary  cells,  as  a  result  of  which  sixty  or  more  carpospores  are 
produced.  An  envelope  of  sterile  cells  grows  up  from  the  base 
and  encloses  this  spore  mass,  forming  the  cystocarp. 

Experiment  has  established  the  fact  that  these  carpospores 
produce  only  tetrasporic  plants,  and  that  the  tetraspores  in  turn 
produce  only  male  or  female  plants.  Thus  a  regular  alternation 
of  sexual  and  non-sexual  individuals  is  set  up.  Cytological 
examination  has  further  proven  that  the  tetrasporic  plants  have 
twice  the  number  of  chromosomes  possessed  by  the  sexual  plants, 
and  there  seems  no  reason  to  doubt  that  we  are  dealing  here 
with  a  true  alternation  of  generations,  the  tetrasporic  plant  (plus 
the  cystocarp)  being  a  sporophyte,  and  the  sexual  plants,  gameto- 
phytes.  This  is  the  more  remarkable  since  all  the  plants  are 
perfectly  similar  in  their  vegetative  structures. 

The  red  algae  are  very  evidently  a  specialized  class  and 
although  they  have  reached  a  marked  degree  of  complexity,  they 
apparently  have  not  been  the  ancestors  of  anything  higher  up  in 
the  evolutionary  series. 

THE  FUNGI 

The  other  great  group  of  the  thallophytes  are  the  fungi, 
which  are  distinguished  from  algae  by  the  absence  of  chlorophyll. 
Their  (Consequent  inability  to  manufacture  food  therefore  compels 
fungi  to  live  either  as  saprophytes  or  as  parasites.  Like  the  algae, 
this  is  a  heterogeneous  group  and  many  of  its  members  seem  more 
closely  related  to  certain  groups  of  algae  than  to  other  fungi,  but 
as  a  matter  of  convenience  and  custom,  and  in  the  absence  of  any 
widely  accepted  "natural"  classification  of  the  thallophytes,  they 
will  all  be  considered  together.  This  immense  arraj^  of  lowly 
plants  is  much  more  numerous  in  specie^  than  the  algae  and 


THE  THALLOPHYTA 


285 


contains  hundreds  of  types  which  are  of  the  utmost  importance  to 
man  and  which  form  the  subject  matter  of  various  special 
sciences,  notably  bacteriology  and  plant  pathology. 

In  the  general  morphology  of  both  their  vegetative  and  their 
reproductive  structures  the  fungi  rather  clearly  parallel  the 
algae,  ranging  from  strictly  unicellular  types  in  the  bacteria, 
through  filamentous  forms,  to  those  which  have  a  large  and 
rather  complex  plant  body;  and   displaying  various  types  of 


Fig.  162.- — Bacteria  of  various  types.  A,  motile  individuals  of  Bacillus 
suhtilis.  B,  non-motile  individuals  of  Bacillus  subtilis.  C,  tetanus  cocci.  D, 
erysipelas  cocci.  E,  pus  cocci.  F,  Spirillum  undula.  G,  gelatinous  or  zodglaea 
condition  of  a  mass  of  bacteria.     H,  cholera  vibrios. 


sexual  reproduction,  both  isogamous  and  heterogamous.  In  all 
fungi  above  the  bacteria,  the  plant  "body  is  composed  of  one  or 
more  filaments,  each  of  which  is  known  as  a  hypha  (plural 
hyphae).  The  whole  mass  of  hyphae,  which  are  often  tangled  or 
matted  together,  is  called  the  mycelium.  Special  absorbing 
organs  or  haustoria  (singular  haustorium)  are  usually  developed, 
through  which  the  plant  draws  its  food  from  the  material  or  sub- 
stratum on  which  it  grows. 

The  series  may  best  be  divided  into  four  classes :  The  Bacteria, 
the  Alga-hke  Fungi,  the  Sac  Fungi  and  the  Basidia  Fungi. 

Bacteria. — These  plants  differ  from  other  fungi  in  being  strictly 
unicellular,  although  they  may  sometimes  form  loose  colonies. 
The  cells  are  usually  very  small,  ranging  from  about  0.025  mm.  to 
as  low  as  0.0005  mm.  in  length,  so  that  bacteria  are  the  smallest  of 
living  things.  Internally,  these  cells  seem  to  be  almost  structure- 
less. There  is  some  evidence,  however,  that  nuclear  material  and 
perhaps  a  vague  nuclear  body  may  be  present,  but  the  very  minute 


286  BOTANY:  PRINCIPLES  AND  PROBLEMS 

size  of  bacteria  makes  a  cytological  study  of  them  exceedingly 
difficult.  The  cell  wall  rarely  contains  cellulose.  One  or  more 
cilia  are  found  in  many  species,  which  thus  have  the  power  of 
active  motility.  Various  structural  types  are  recognized  by 
bacteriologists,  of  which  the  commonest  are  the  coccus  form, 
which  is  spherical;  the  bacterium  or  bacillus,  which  is  rod-shaped, 
and  the  spirillum,  which  is  curved  (Fig.  162).  The  only  type  of 
reproduction  known  is  simple  cell-division,  or  fission,  which  is 
unaccompanied  by  any  of  the  complex  phenomena  found  in  the 
higher  plants,  but  which  in  the  presence  of  an  abundant  food 
supply  may  take  place  so  frequently  that  a  single  cell  will  give 
rise  to  milhons  of  individuals  in  a  day's  time.  At  the  onset  of 
unfavorable  conditions  the  protoplasm  of  the  cell  draws  itself 
together  and  produces  a  thick-walled  spore  which  will  germinate 
whenever  a  favorable  environment  again  ensues.  Actively  grow- 
ing bacteria  will  ordinarily  withstand  relatively  high  tempera- 
tures but  their  resting  spores  are  still  more  resistant  and  will  often 
be  found  alive  and  able  to  germinate  after  hours  of  subjection  to 
boiling  water.  They  can  also  survive  extreme  cold  and  dryness. 
In  their  various  characteristics,  therefore,  the  bacteria  (as  we 
have  before  noted)  show  much  resemblance  to  the  blue-green 
algae. 

Although  bacteria  occur  in  countless  myriads  of  individuals 
and  although  their  diverse  activities  make  them  almost  omnipre- 
sent in  air,  water,  and  soil,  they  are  so  small  as  to  be  quite  invisible 
and  from  a  practical  point  of  view  would  be  entirely  negligible 
were  it  not  for  the  profound  effects  which  they  produce.  Their 
importance  in  agriculture,  particularly  through  their  activity 
in  the  soil  and  in  dairy  products ;  their  role  as  the  chief  agents  of 
fermentation  and  decay,  and  particularly  their  tremendous  direct 
interest  to  man  as  the  cause  of  some  of  the  worst  and  most  preva- 
lent of  those  diseases  which  afflict  him  and  his  domestic  animals 
and  plants,  have  caused  them  to  be  studied  with  especial  thorough- 
ness and  have  established  bacteriology  as  one  of  the  most  active 
of  the  sciences. 

Our  knowledge  of  bacteria  dates  only  from  the  latter  half  of 
the  nineteenth  century.  Their  existence  was  proven  by  that 
great  Frenchman,  Louis  Pasteur  (Fig.  163),  who  labored  for 
years,  meeting  with  the  ridicule  and  antagonism  which  often 
greets  a  new  discovery,  before  he  could  convince  his  fellow 
scientists  that  such  minute  objects  were  actually  alive  and  were 


THE  TIIALLOPHYTA  287 

responsible  for  so  many  of  the  happenings  of  daily  life.  Since 
Pasteur's  day,  as  the  result  of  our  knowledge  of  bacteria,  the 
practice  of  medicine  (and  to  a  considerable  extent  that  of  certain 
branches  of  agriculture  and  industry)  has  been  radically  changed; 
and  the  astonishing  advances  in  modern  surgery,  made  possible 
by  Pasteur's  discoveries  and  inaugurated  by  the  great  surgeon 
Lister,  have  converted  this  branch  of  medicine  from  a  dreaded 


Fig.   163. — Louis  Pasteur,  1822-1895. 

last  resort  to  a  common  and  safe  means  of  relief.  The  bacter- 
iologist of  today  has  developed  a  complicated  and  elaborate 
technique  whereby  he  can  isolate  individual  bacteria,  cultivate 
them  artificially  on  especially  prepared  and  sterilized  foods  or 
media  of  various  kinds,  and  study  the  characteristic  appearance 
of  individuals  and  masses,  together  with  the  physiological  behav- 
iour peculiar  to  each.  Both  as  the  enemies  and  as  the  alhes  of 
the  human  race,  these  lowly  plants  with  which  the  bacteriologist 
works  are  among  the  most  important  members  of  the  vegetable 
kingdom. 

Saprophytic  Types. — The  saprophytic  bacteria  live  on  dead 
plant  or  animal  material.  They  are  responsible  for  much  of  the 
fermentation  which  carbohydrate  substances  often  undergo  when 
exposed  to  the  ah-  and  which,  as  a  form  of  incomplete  respiration, 


288  BOTANY:  PRINCIPLES  AND  PROBLEMS 

we  have  discussed  in  our  study  of  metabolism.  Decay,  essen- 
tially the  same  type  of  process  as  fermentation  except  that  it 
takes  place  in  all  sorts  of  organic  substances  and  is  carried  through 
to  a  complete  break-down  of  these  substances  into  carbon  dioxide, 
water,  nitrogen,  and  mineral  salts,  is  brought  about  almost  entirely 
through  the  activity  of  the  many  types  of  putrefactive  and 
decay-producing  bacteria.  All  successful  methods  of  preserving 
food  depend  on  eliminating  these  bacteria  or  preventing  their 
activity. 

Pathogenic  Types.- — Those  members  of  the  class  which  are 
parasitic  on  other  organisms  are  known  as  pathogenic  bacteria. 
A  particular  species  is  the  cause  of  each  of  the  various  bacterial 
diseases,  such  as  diphtheria,  tuberculosis,  typhoid  fever,  pneu- 
monia, cholera,  and  many  others  among  animals  and  man,  as 
well  as  pear  blight,  cucumber  wilt,  black  rot  of  cabbage,  and  others 
among  plants.  These  diseases  are  often  communicated  from  one 
individual  to  another,  since  the  pathogenic  bacteria  responsible 
for  them  may  be  easily  transmitted  through  air,  water,  food,  or 
contact.  Bacteria  which  are  very  minute  or  otherwise  difficult 
to  recognize  are  also  probably  responsible  for  many  diseases 
the  cause  of  which  is  at  present  unknown. 

The  harmful  effect  of  pathogenic  bacteria  on  animals  is  often 
not  due  to  the  direct  attack  of  the  parasite  but  to  the  poisonous 
by-products,  or  toxins,  which  they  secrete  and  which  enter  the 
blood.  The  afflicted  individual  will  often  produce  antitoxins 
capable  of  counteracting  the  poisonous  effect  of  the  toxins  and 
of  rendering  the  individual  immune  for  a  time  to  the  attacks  of 
this  particular  parasite.  The  practice  of  vaccination  consists  in 
inoculating  an  individual  with  the  parasitic  bacteria  and  thus 
subjecting  him  to  a  mild  attack  of  the  disease  in  order  to  stimulate 
the  production  of  sufficient  antitoxin  in  his  system  to  confer 
immunity  upon  him  for  a  long  time.  Vaccination  is  particularly 
effective  against  small  pox  and  typhoid  fever.  The  attacks 
of  certain  other  disease-producing  organisms  may  also  be  pre- 
vented or  rendered  less  virulent  by  injecting  into  the  circulation 
a  little  blood  serum  from  an  individual  (usually  a  cow  or  horse) 
which  has  had  the  disease  and  whose  blood  is  therefore  rich  in 
antitoxin.  This  serum  or  antitoxin  treatment  has  been  especially 
successful  with  diphtheria,  tetanus,  and  hog  cholera. 

In  our  study  of  the  soil  we  mentioned  two  other  groups  of 
bacteria  which  are  of  especial  importance  to  the  higher  plants 


THE  THALLOPIIYTA 


289 


because  of  their  relation  to  nitrogen  (Fig.  18).  These  are  the 
7iitrogen-fixing  bacteria,  which  are  found  in  tubercles  on  the  roots 
of  plants  belonging  to  the  Legume  family  and  which  are  able  to 
take  nitrogen  directly  from  the  air  and  to  incorporate  it  into 
their  bodies;  and  the  nitrifying  bacteria  which  convert  ammonia, 
the  end  product  of  protein  decay,  into  nitrite  salts  and  these,  in 
turn,  into  nitrate  salts,  the  only  form  in  which  nitrogen  can 
generally  be  utihzed  by  green  plants. 

Phycomycetes  or  Alga-like  Fungi.^These  fungi,  as  their  name 
implies,  resemble  rather  closely  certain  of  the  algae,  particularly 


Fig.  164. — The  bread-mold  (Rhizopus).  General  habit  of  the  plant.  The 
mycelium  (A)  of  much-branched  hyphae  penetrates  the  substratum  and  sends  up 
into  the  air  stouter  hyphae,  the  sporangiophores  (B),  on  which  are  borne  the 
spherical  sporangia  (C).  One  sporangium  has  burst,  shedding  its  spores,  a  few  of 
which  still  adhere  to  the  central  axis  or  columella  of  the  sporangium.  The  two 
groups  of  sporangiophores  are  connected  by  a  stolon  (D) . 

in  their  methods  of  reproduction.  They  seem  especially  near 
the  Siphonales  because  of  the  fact  that  their  filaments  (hyphae) 
are  coenocytic,  and  it  is  probably  from  that  general  region  of 
the  algal  series  that  they  originated. 

The  Phycomycetes  include  a  great  many  of  the  forms  which  we 
know  as  molds  and  blights.  Three  orders  are  particularly  notable 
and  we  shall  describe  them  briefly.  These  are  the  Mucorales, 
the  Saprolegniales,  and  the  Peronosporales. 


290 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


1.  Mucorales,  the  True  Molds  or  Black  Molds  (Fig.  164).— 
These  fungi  are  very  common  on  moist,  decaying  organic  material 
such  as  manure,  rotting  fruit,  and  the  hke.  Their  white,  cob- 
webby mycehum,  composed  of  much-branched  hyphae,  freely 
penetrates  the  substratum  in  all  directions.  From  the  surface, 
stout  hyphae  arise  into  the  air  and  bear  at  their  tips  globular 


Fig.  165. — Mucor.  Formation  and  germination  of  the  zygospore.  A,  two 
conjugating  hyphae  in  contact.  B,  a  gamete,  a,  has  been  cut  off  from  the  end  of 
each  hypha.  C,  the  enlarged  gametes.  D,  thick-walled  zygospore  which  has 
arisen  from  a  union  of  the  two  gametes.  E,  zygospore  germinating  to  form  a 
sporangium.      {From  Slrasburger,  after  Brefeld). 


sporangia  which  burst  and  liberate  masses  of  dark-colored 
spores,  each  of  which  may  germinate  directly  into  a  new  plant. 
Sexual  reproduction  is  isogamous  or  essentially  so  (Fig.  165). 
Two  short  branches  or  suspensors,  arising  from  adjacent  hyphae, 
approach  one  another  and  come  into  contact  end-to-end.  From 
the  tip  of  each  is  cut  off  a  single  multinucleate  cell  which  is  the 
gamete,  and  these  two  adjacent  gametes  fuse  to  produce  a  thick- 
walled  zygospore.  It  has  been  found  that  two  distinct  sexual 
strains  often  exist,  entirely  similar  outwardly  but  functioning 
quite  differently  in  reproduction.  These  have  been  called  the 
plus  and  the  minus  strains  and  correspond  to  the  two  sexes,  for 


THE  THALLOPHYTA 


29] 


zygospores  arc  usually  produced  only  when  a  plus  plant  and 
a  viinus  plant  come  into  contact.  These  fungi  furnish  an 
interesting  example  of  a  physiological  differentiation  of  sex 
which  is  not  accompanied  by  morphological  differences. 

2.  Saprolegtiiales  or  Water  Molds  (Fig.  16G).— In  contrast 
to  the  Mucorales,  this  group  is  entirely  aquatic.  Its  members 
live  on  the  bodies  of  dead  insects  and  other  animals,  and  a  few 
are  parasitic,  attacking  fish  and  amphibians.     A  cell  cut  off  from 


Fig.  166. — Saprolegnia,  one  of  the  Saprolegtiiales.  A,  mycelium  on  an  insect 
in  water.  B,  terminal  sporangium  producing  zoospores.  C,  zoospores.  D, 
sexual  organs;  the  two  oogonia  each  containing  several  eggs.  The  one  at  the 
left  penetrated  by  antheridial  filaments. 


the  tip  of  a  hypha  develops  into  a  sporangium  and  liberates  a 
large  number  of  motile  zoospores.  Sexual  reproduction  is  heter- 
ogamous.  A  single-celled  spherical  oogonium  is  developed 
and  produces  several  eggs  which  are  fertilized  by  the  contents 
of  an  antheridial  filament,  a  slender  branch  which  grows  out  from 
the  main  hypha  near  the  oogonium  and  fuses  with  it.  In  many 
cases  the  egg  develops  into  mature  spores  without  having  been 
fertilized  at  all. 

Peronosporales  or  Blighls   and   Downy  Mildcia-i   (Fig.    167). — 
The  species  composing  this  group  arc  all  parasites  on  the  higher 


292 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


plants.  Their  spores  germinate  on  the  surface  of  the  leaf  or 
stem  and,  entering  the  tissues,  branch  through  them  in  all  direc- 
tions, absorbing  food  and  often  causing  the  death  of  the  host 
plant.  Definite  fruiting  bodies  are  produced  at  the  surface  of 
various  organs  of  the  host,  where  specialized  hyphae  become 
constricted  to  form  spores,  which  separate  and  are  carried  away 


Fig.  167. — ^Zfewgo,  one  of  the  Peronosporales.  A,  section  through  a  leaf-blade 
showing  portion  of  a  "blister"  produced  by  the  fungus.  The  hyphae  penetrate 
between  the  cells  of  the  leaf,  sending  into  them  small,  sucker-like  structures. 
Under  the  epidermis  are  produced  rows  of  conidia.  B,  sexual  organs,  the  anther- 
idial  filament  (at  right)  discharging  a  male  nucleus  into  the  oogonium,  at  left. 

by  the  wind.  Non-sexual,  aerial  spores  produced  in  this  manner 
are  called  conidia,  and  are  of  frequent  occurrence  among  fungi. 
Sexual  organs  appear  in  the  deeper  tissues.  The  egg  nucleus  in 
the  oogonium  is  fertilized  by  a  male  nucleus  from  an  adjacent 
antheridial  filament,  and  a  thick-walled  oospore  is  formed. 
Such  destructive  plant  parasites  as  the  potato  blight  and  the 
grapevine  mildew  belong  to  this  order. 

Ascomycetes  or  Sac  Fungi. — This  enormous  group  of  plants 
includes  over  20,000  species.  They  show  but  little  resemblance 
to  the  algae,  and  although  all  must  have  come  originally  from 
some  chlorophyll-bearing  forms,  their  exact  ancestry  is  not  clear. 
These  fungi  are  typically  land-inhabiting  plants  and  include 
both  saprophytic  and  parasitic  species.  Both  groups  differ 
from  the  Phycomycetes  in  the  fact  that  their  hyphae  are  divided 
into  cells  by  cross-walls,  and  that  sexual  processes  are  much 
reduced  or  altogether  lacking.     The  plant  body  commonly  consists 


THE  TIIALLOPIIYTA 


293 


of  a  much-branched  mycelium  extending  throughout  the  sub- 
stratum, and  a  definite  fruiting  body  which  is  developed  at  the 
surface.  Each  group  displays  a  rather  specialized  method  of 
spore  formation. 

The  Ascomycetes  are  distinguished  by  their  production  of 
spore  sacs  or  asci  (singular,  ascus)  in  each  of  which  are  borne 
eight  spores,  the  ascospores  (Fig.   168).     A  group  of  asci  are 


Fig.  168. — Spore  production  in  an  ascomycete.  Portion  of  the  hymenium, 
or  fruiting  surface,  of  Peziza,  showing  the  asci,  each  with  eight  ascospores. 
Among  the  asci  are  slender,  sterile  hairs,  or  paraphyses,  and  two  young  asci. 

generally  embedded  in  a  mass  of  sterile  hj^phae  and  partially  or 
completely  surrounded  by  a  protective  envelope  of  compact 
mycelium.  Such  a  fruiting  body  is  known  as  an  ascocarp.  In 
many  cases,  this  ascocarp  has  been  found  to  originate  as  the 
result  of  a  sexual  union  deep  in  the  mycelium,  the  whole  process 
bearing  a  considerable  resemblance  to  that  found  among  the  red 
algae.  The  ascomycetes  include  an  immense  variety  of  types, 
only  a  few  of  which  can  be  mentioned  here. 

1.  Pezizales  or  Cup  Fungi  (Fig.  169). — Throughout  this  order 
the  ascocarp  is  a  broad  disc,  cup,  or  funnel,  and  the  name  Dis- 


294 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


comycetes  or  disc  fungi  is  therefore  often  applied  to  the  group. 
The  inner  surface  of  the  cup  consists  of  a  layer  of  asci  and  sterile 
hyphae.  Such  a  layer  is  known  as  the  hymeniimi.  These  plants 
are  almost  all  saprophji:es  and  their  brightly  colored  fruiting 


Fig.   169. — Peziza  ciliata. 


The  cup-like  ascocarps,  on  the  inner  surface  of  which 
the  asci  are  borne. 


bodies  are  conspicuous  on  rotting  legs,  damp  earth,  and  similar 
situations. 

Related  to  the  cup  fungi  are  the  Morels  {Helvellales),  which 
develop  large,  fleshy,  and  edible  fructifications,  the  fruiting 
surface  of  which  is  broken  up  into  irregular  pockets;  and  the 
Truffles  {Tuber ales),  which  produce  subterranean,  tuber-like 
ascocarps,  usually  associated  with  the  roots  of  oak  trees,  and 
much  prized  as  delicacies. 


Fig.  170.- — Plowrightia  morbosa,  the  "black  knot"  of  plums  and  cherries. 
Hard,  black  fruiting  mass  of  the  fungus  on  a  twig  which  it  has  attacked.  The 
small  dots  are  openings  to  the  perithecia. 


2.  Pyrenomycetes  or  Black  Fungi. — Here  are  found  a  large 
and  varied  group  of  fungi  which  include  both  parasitic  and  sapro- 
phytic species.  Their  mycelium  is  often  hard  and  compact  and 
is  characteristically  dark  in  color.  The  ascocarp  consists  of  a 
very  small,  flask-shaped  structure,  the  peritheciu7n,  lined  with 


THE  THALLOPIIYTA 


295 


a  hymenium  and  opening  to  the  air  by  a  minute  pore.  Here 
belong  the  knot  and  wart  fungi  found  on  so  many  woody  plants, 
many  of  which,  such  as  the  "black  knot"  of  plums  (Fig.  170), 
are  serious  parasites.  In  this  order  also  occur  the  destructive 
chestnut  bark  fungus  and  other  important  disease-producing 
organisms. 

3.  Perisporiales    or  Mildews. — These    small  fungi   produce  a 
cobweb-like  mycelium  which  spreads  over  the  surface  of  the 


Fig.  171. — Aspergillus  {A)  and  Peni- 
cillium  (B).  Hyphae  bearing  chains  of 
air-spores  or  conidia.      (From  Strasburger). 


Fig.  172. — Yeast  (Saccharo- 
myccs).  Single-celled  plants  in 
various  stages  of  division  by 
budding. 


leaves  of  many  plants  and  is  parasitic  on  their  epidermal  cells. 
Conidia  are  produced  in  abundance.  Toward  the  end  of  the 
season,  as  the  result  of  a  sexual  union  between  female  branches 
(ascogonia)  and  antheridia,  there  are  developed  a  host  of  minute, 
dark,  globular,  and  hard-walled  ascocarps  or  perithecia.  These 
are  filled  with  asci,  and  on  breaking  open  the  next  spring,  release 
the  ascospores. 

4.  Plectascales  the  Blue  and  Green  Molds  (Fig.  171). — Here  are 
found  the  common  molds  (aside  from  the  Mucorales)  which  appear 
on  bread,  cheese,  leather,  and  almost  all  organic  substances  which 
will  "mold"  when  subjected  to  dampness.  Their  abundant 
masses  of  conidia  arc  typically  greenish  or  bluish  in  color.  Small, 
rounded  ascocarps,  full  of  irregularly  scattered  asci  and  lacking  a 
hymenium,  are  occasionally  produced.  No  members  of  the  order 
are  parasitic,  but  a  species  of  PeniciUium  is  of  economic  impor- 
tance as  responsible  for  the  peculiar  flavor  of  Roquefort  cheese. 

5.  Saccharomycetes  or  Yeasts  (Fig.  172). — These  minute  plants 
are  usually  included  within  the  ascomycetes.     The  individual  is  a 


296 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


single  cell,  possessing  a  definite  nucleus,  cytoplasm,  and  sap-cavity 
and  producing,  by  the  process  of  "budding",  a  loose,  irregular 
chain  of  cells.  When  conditions  become  unfavorable,  the  con- 
tents of  some  of  the  cells  divides  into  four  spores,  forming  a 
structure  which  is  thought  to  represent  a  modified  ascus.  The 
yeasts  are  saprophytes  on  sugary  substances,  thriving  in  the 
absence  of  free  oxygen,  and  are  the  chief  agents  in  alcoholic 
fermentation,  a  process  which  has  been  described  in  a  preceding 
chapter.     Their  use  in  the  raising  of  bread  is  familiar  to  everyone. 


Fig.  173. — Spore  production  in  a  basidiomycete.  ^,  cross  section  of  a  few  of 
the  gills  of  a  mushroom.  B,  a  portion  of  the  surface  of  one  of  the  gills,  much 
enlarged.  The  stout  cells  are  basidia.  Each  of  them  bears  four  basidiospores  on 
slender  stalks  or  sterigmata.     The  lowest  basidiuna  has  shed  its  spores. 

Basidiomycetes  or  Basidia  Fungi. — Like  the  Ascomycetes, 
this  is  a  very  large  and  varied  group  containing  over  20,000 
species.  Its  specialized  reproductive  structure,  the  basidium 
(Fig.  173),  is  the  swollen  terminal  cell  of  a  hypha  and  typically 
bears  four  basidiospores,  each  supported  on  a  delicate  stalk 
or  sterigma  (plural  sterigmata).  The  basidia  are  arranged  in  a 
more  or  less  definite  hymenium.  Aside  from  this  difference  in  the 
method  of  spore  production,  the  group  as  a  whole  is  distinguished 
from  the  Ascomycetes  by  the  almost  complete  absence  of  sexual 
processes  and  by  the  larger  and  more  conspicuous  fruiting  bodies. 
The  Basidiomycetes  are  regarded  as  the  highest  of  the  fungi 
and  are  believed  to  have  come  from  Ascomycetes,  the  basidium 
representing  a  much  modified  ascus. 

A  few  orders,  notably  the  Smuts  (Ustilaginales)  and  the  Rusts 
(Uredinales)  differ  from  the  typical  Basidiomycetes  (or  Auto- 
basidioniycetes)  in  having  basidia,  or  structures  thought  to  repre- 


THE  THALLOPHYTA  297 

sent  basidia,  which  instead  of  being  one-celled  are  composed  of 
three  or  four  cells,  each  producing  a  spore. 

1.  Ustilaginales  or  Smuts. — ^Here  are  found  a  number  of 
destructive  parasites  which  attack  floral  organs,  especially  among 
members  of  the  Grass  family.  The  mycelium  spreads  through 
the  body  of  the  host  plant  and  at  flowering  time  gathers  in  dense 
masses,  particularly  in  the  ovaries  and  surrounding  tissues,  which 
become  swollen  and  distorted,  and  form  the  so-called  smut. 
The  mycelium  here  becomes  transformed  directly  into  a  mass  of 
black,  thick-walled  spores  which  survive  the  winter.  On  germin- 
ating the  next  spring,  each  spore  produces  a  short  filament  of 
three  or  four  cells,  the  promycelium.  This  promycelium  is 
thought  to  represent  a  basidium,  for  each  cell  bears  one  or  more 
spores  or  sporidia,  which  are  capable  of  infecting  new  plants.  The 
smuts  of  corn,  oats,  and  wheat  are  particularly  destructive. 

2.  Uredinales  or  Rusts. — All  members  of  this  order  are  para- 
sites, often  becoming  very  destructive,  and  have  the  most  compli- 
cated life  histories  of  any  of  the  fungi. 

The  common  wheat  rust,  Puccinia  graminis  (Fig.  174),  is  the 
best  known  member  of  the  group.  The  mycelium  of  this  species, 
in  the  tissues  of  the  wheat  plant,  breaks  through  the  surface 
and  produces  clusters  of  one-celled,  reddish,  rough-walled  spores, 
the  summer  spores  or  uredospores,  which  give  a  rust-like  appear- 
ance to  the  stems  and  leaves.  Each  uredospore  may  germinate 
directly  on  another  wheat  plant  and  produce  a  new  mycelium. 
Toward  the  end  of  the  season,  clusters  of  another  type  of  spore 
are  produced  at  the  surface  of  the  plant.  These  are  black,  two- 
celled  and  heavy-walled,  and  are  known  as  winter  spores  or 
teleutospores.  They  five  over  the  winter  and  in  the  following 
spring  each  cell  germinates  into  a  slender,  four-celled,  saprophytic 
promycelium  somewhat  as  in  the  smuts.  Each  of  the  cells  here 
produces  but  a  single  sporidium,  however,  so  that  the  structure 
displays  a  much  closer  resemblance  to  a  typical  basidium.  The 
remarkable  fact  has  been  demonstrated  that  these  sporidia 
(or  basidiospores)  do  not  infect  wheat,  but  will  attack  only 
plants  of  the  barberry.  The  spores  germinate  on  the  barberry 
leaves,  penetrate  the  tissues,  and  produce  there  a  vigorous  mycel- 
ium. On  the  upper  surface  of  the  leaf  soon  appear  small  flask- 
shaped  structures,  the  spermagonia,  which  produce  enormous 
numbers  of  very  minute  cells  or  spermatia.  As  its  name  implies, 
this  organ  has  been  thought  to  represent  the  male  sexual  appa- 


298 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


ratus.  Whatever  its  original  share  in  the  Hfe  cycle  may  have 
been,  however,  it  seems  now  to  be  entirely  functionless.  On  the 
lower  surface  of  the  barberry  leaves  are  formed  cluster-cups  or 


Fig.  174. — Puccinia  graminis,  the  wheat  rust.  A,  wheat  leaf  with  groups  of 
summer  spores  (uredospores).  B,  uredospores  (one  teleutospore  among  them). 
C,  germinating  uredospore.  D,  wheat  stem  with  masses  of  winter  spores  (tel- 
eutospores).  E,  two  teleutospores.  F,  germinating  teleutospore,  producing  a 
promycelium  on  which  are  borne  the  sporidia.  G,  sporidium  germinating  on 
epidermis  of  barberry  leaf.  H,  lower  surface  of  barberry  leaf,  showing  cluster- 
cups  or  aecidia.  /,  section  through  the  leaf  of  barberry  showing  spermagonium 
on  upper  surface  and  aecidium  on  lower.  The  aecidium  is  producing  masses  of 
aecidiospores,  which  infect  wheat  plants.     {B,  C,  F  and  G  after  De  Bary). 

aecidia  (singular,  aecidium),  flaring,  cup-shaped  structures  from 
the  base  of  which  arise  long  rows  of  aecidiospores.     These,  in 


THE  THALLOPIIYTA 


299 


turn,  never  reinfect  barberry  but  attack  only  wheat  plants,  thus 
completing  the  life  cycle. 

Many  other  rusts  resemble  this  species  in  alternating  between 
two  distinct  hosts,  but  some  are  confined  entirely  to  one.  The 
rusts  produce  many  serious  plant  diseases,  among  them  the  white 


Fig.  175. — Agaricus  campestris,  the  common  mushroom.  Views  of  youns 
and  of  mature  fructifications.  The  broad,  expanded  cap  or  pilius  bears  the  gills 
on  its  lower  surface  and  is  supported  by  the  stalk  or  stipe.  The  young  fructifica- 
tion, at  left,  is  surrounded  by  a  membrane  or  volva,  which  breaks  as  the  pileus 
expands,  and  the  remains  of  which  can  be  seen  as  a  ring  around  the  stipe. 

pine  blister  rust,  which  has  its  uredo-stage  on  currants  and 
gooseberries;  the  apple  rust,  which  has  its  teleuto-stagc  on  red 
cedar,  and  many  others. 

The  typical  or  true  Basidiomycetes,  or  Autohasidiomycetes, 
include  two  main  sub-classes;  the  Hymenomycetes,  in  which 
the  fruiting  surface  or  hymcnium  is  exposed  to  the  air,  and 
the  Gasteromycetes,  in  which  it  is  enclosed  within  the  tissue  of  the 


300 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


fructification.  These  are  both  chiefly  saprophytic,  the  extensive 
mycehiim  penetrating  deeply  and  under  favorable  conditions 
producing  at  the  surface  very  characteristic  fructifications  or 
sporophores. 

A.  Hymenomycetes. — These  fungi  are  divided  into  a  number  of 
orders,  but  by  far  the  most  important  are  the  Agaricales  or 
agarics,  which  include  the  mushrooms  and  toadstools.     This  is  a 


Fig.  176. — A  gill  fungus  growing  in  a  hollow  tree.  The  mycelium  penetrates 
the  wood  in  all  directions  and  has  produced  the  fruiting  body  here  shown.  In 
cases  like  this  it  is  sometimes  hard  to  tell  whether  the  fungus  is  a  parasite  or  a 
saprophyte. 

very  large  group  and  includes  the  most  important  of  the  so-called 
"fleshy  fungi".  Because  of  the  edibility  of  the  sporophore  of 
many  of  its  members  and  the  poisonous  character  of  a  few  of 
them,  the  Agaricales  have  received  intensive  study  and  are  per- 
haps more  widely  known  than  any  other  fungi. 

The  order  is  divided  into  a  number  of  families  according  to 
the  type  of  fruiting  body  produced.  In  the  Agaricaceae  or  gill 
fungi  (Figs.  175,  176,  and  5)  which  are  the  mushrooms  and  toad- 
stools proper,  the  hymenium  covers  the  surface  of  plate-like 
structures  or  gills.  The  sporophore  in  this  group  is  the  typical 
"toadstool",  which  consists  of  a  stalk  or  stipe  supporting  a  broad 


THE  TlIALLOPnvrA 


301 


cap  or  pileus,  from  the  under  surface  of  which  hang  the  gills.  A 
few  of  the  species  arc  regularly  cultivated  and  constitute  a  food 
product  of  sonic  importance.     A  number  are  also  poisonous,  and 


Fig.  177. — A  pore  fungus.  Bnickot-liko  fructification  of  one  of  the  specie 
which  grows  on  tree-trunk.s.  /I,  lower  surface,  showing  pore  openings.  B,  upper 
surface.  C,  section  through  a  part  of  the  fructification,  showing  the  long,  tube- 
like pores,  on  the  inner  surfaces  of  which  the  spores  are  borne.  This  type  of 
fructification  adds  a  new  layer  each  year,  and  two  such  annual  layers  are  here 
evident. 


-Puff-balls.     Cleneral  api)carance  of  young  and  of  mature 
fructifications. 


the  amateur  mushroom  collector  is  always  in  danger  of  adding 
them  to  his  menu.  In  the  polypores  (Polyporaceae,  Fig.  177)  the 
hymenium  lines  narrow  tubes  which  open  by  pores  on  the  surface 
of  the  pileus.     These  include  many  mushroom-like  types  as  well 


302 


HOT  AX  V:  PRINCIPLES  AND  PROBLEMS 


as  the  hard,  bracket-Hke  forms  so  common  on  rotting  logs,  which 
are  very  destructive  of  wood  and  are  sometimes  parasites  upon 
Hving  trees.  In  the  Tooth  fungi  (Hydnaceae)  the  hymenium 
covers  the  surface  of  tooth-hke  or  spine-hke  projections. 

Close  to  the  Agaricales  are  the  Coral  fungi  (Clavariales,  Fig. 
179)  the  much-branched  sporophores  of  which  are  covered  by  the 
hymenium. 

B.  G aster omycetes. — These  arc  the  highest  of  the  fungi.  Their 
fructification  is  in  general  a  rounded  mass  of  hyphae,  the  outer- 


FlG, 


)ove,  and  young  puff-balls,  below. 
In  the  photograph  are  also  ferns,  lycopods,  and  leafy  liverworts. 


most  layer  of  which  becomes  differentiated  into  a  cortex  or 
peridium,  surrounding  the  inner  mass  of  hyphae  and  basidia 
which  is  called  the  gleha.  Two  important  orders  are  the  Lyco- 
perdales  and  the  Phallales. 

1.  Lycoperdales  or  Puffballs  (Figs.  178  and  179). — The  sporo- 
phore  is  here  a  globular  structure  which  often  becomes  very  large 
and  is  usually  edible  when  young.  At  maturity  the  peridium 
breaks  open  at  the  top  and  a  mass  of  dark  spores  is  discharged 
therefrom. 

2.  Phallales  or  Stink-horns  (Fig.  180). — The  gleba  here  ripens 
into  a  foul-smelling  mass,  breaks  through  the  peridium,  and  is 


THE  TI/ALLOPUYTA 


303 


pushed  upward  at  the  end  of  a  stalk.  Tlic  rank  odor  a1  tracts 
to  those  fungi  many  carrion-loving  insects. 

Lichens.  In  addition  to  the  algae  and  fungi,  the  thallophytcs 
include  a  remarkable  group  of  composite  plants,  the  Uchem. 
The.se  are  fungi  in  the  mycelium  of  which  groups  of  algal  cells  are 
entangled  (Fig.  181).  The  advantage  to  the  fungus  of  this 
intimate  association  is  evident,  and  the 
alga  is  also  probably  benefited  to  some 
extent.  Instead  of  regarding  this  as  a 
case  of  true  symbiosis,  however,  most 
botanists  look  upon  the  fungus  as  parasitic 
on  the  algal  member  of  the  combination, 
even  though  this  parasitism  is  very  mild. 
The  two  component  plants  have  been  arti- 
ficially separated,  and  it  has  been  found 
that  the  alga  can  exist  independently  in 
every  case  but  that  the  fungus  is  unable  to 
do  so.  Lichens  have  also  been  "synthe- 
sized" experimentally  by  bringing  appro- 
priate algae  and  fungi  together.  The 
fungus  members  of  lichens  are  almost 
always  ascomycetes.  A  number  of  different 
algae  are  represented  in  lichens  but  these 
belong  mainly  to  the  Cyanophyceae  and  to 
the  Protococcales.  All  are  very  simple  in 
character.  The  fungus  mycelium  is  often 
somewhat  gelatinous  and  thus  tends  to 
absorb  water  readily  and  hold  it  tenaciously. 
It  is  much  more  compact,  differentiated,    the  stink-horn.    Fructi- 

1  J.1  i.i,j.r        J-  fication,      showing      the 

and  dennite  in  shape  than  that  oi  ordmary  gieba,  above  at  the  end 
fungi  and  is  essentially  a  flat  thallus  (Fig.    of  the    stalk,  and    the 

ioo\        rrii.  •        X  r     XI     11  remains  of  the  pendium 

182).     Ihree    mam   types    of   thallus    are    below, 
recognized:  The   crustaceous,  which   grows 

closely  appressed  to  the  surface  of  rocks,  bark  and  similar  struc- 
tures; the  foliose,  in  which  the  thallus  is  broader  and  somewhat 
branched,  suggesting  that  of  the  liverworts;  and  the  fruticose,  in 
which  it  is  slender  and  vcM-y  much  brancluMJ  and  may  eitluM'  he 
erect  or  hanging. 

Asexual  multiplication  is  accomplisluHl  by  the  production  and 
dispersal  of  soredid  or  small  l)its  of  mycelium  in  which  a  few  algal 
C(>lls  are  entangle*!.     The  fructifications  are  conspicuous,  and  the 


Fig.   lSO.~Phal(us, 


304 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


c 


Fig.  181. — Physcia  stcllaris,  a  lichen.  A,  section  through  an  ascocarp,  show- 
ing fruiting-surface  (hymenium)  of  asci  and. sterile  hairs.  B,  a  portion  of  the 
hymenium  enlarged,  showing  two  asci  and  several  paraphyses.  C,  section 
through  the  thallus,  showing  the  mass  of  mycelial  filaments  and  (near  upper 
surface)  a  group  of  algal  cells  embedded  in  the  mycelium. 


Fig.    182. — A  lichen  (Parmeiia),  showing  the  rather  thin  and  thallus-like  charac- 
ter of  the  plant  body  and  the  cup-shaped  sac-fruits  or  ascocarps. 


THE  THALLOPIIYTA  305 

ascocarps  are  usually  either  cup-shaped  or  disc-shaped,  as  in  tlie 
Discomycetes  (Fig.  181).  Definite  and  functional  sex  organs, 
somewhat  suggestive  of  those  in  the  red  algae,  have  been  demon- 
strated in  certain  species. 

Lichens  are  usually  xerophytic  and  will  thrive^  on  hare  rocks 
and  exposed  places  where  no  other  vegetation  can  exist.  Their 
importance  in  the  economy  of  nature  is  therefore  considerable, 
but  they  include  only  a  few  species  which  arc  of  direct  pra(!tical 
value  to  man. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

669.  The  thallophytes  are  the  most  varied  of  all  the  four  main  divi- 
sions of  the  plant  kingdom.     Can  you  suggest  why  this  is  so? 

670.  In  what  way  is  the  classification  of  the  thallophytes  given  in  this 
book  an  "artificial"  one? 

671.  All  the  groups  of  algae  are  found  in  the  ocean,  Imt  few  in  fresh 
water.     Explain 

672.  Of  what  advantage  is  the  possession  of  motility  to  the  zoospores 
of  the  algae? 

673.  Zoospores  of  algae  will  generally  swim  toward  the  light  rather 
than  away  from  it.     Of  what  advantage  is  this  to  the  plant? 

674.  Unicellular  algae  are  frequently  spherical  in  form,  but  among 
larger  tj'pes  the  body  is  never  a  solid  sphere.     Explain. 

675.  Algae  are  in  general  much  more  filamentous  and  finely  branched 
than  land  plants.     Explain. 

676.  Algae  which  are  completely  and  somewhat  deeply  submersed 
all  the  time  are  much  more  finely  branched  and  dissected  than  those 
which  are  near  the  surface  or  are  partially  exposed  to  the  air  for  some 
of  the  time.     Explain. 

677.  Are  algae  commoner  on  a  rocky  coast  or  on  a  sand}'  one?  J']x- 
plain. 

678.  Seaweeds  wliich  grow  between  tide  marks  are  often  very  gela- 
tinous.    Explain. 

679.  The  "plankton"  is  more  abundant  in  the  cooler  parts  of  the 
ocean  than  in  tlie  warmer  ones.     Why? 

680.  The  largest  of  the  algae  are  usually  found  in  cool  northern  seas 
rather  than  in  the  warm  tropical  ones.     Why? 

20 


306  BOTANY:  PRINCIPLES  AND  PROBLEMS 

681.  There  are  no  living  algae  or  other  green  plants  at  depths  greater 
than  a  few  hundred  feet  below  the  surface  of  the  ocean.     Why? 

682.  What  factors  may  affect  the  dcptli  t(j  which  algae  will  grow  in 
the  ocean? 

683.  Since  no  green  plants  live  in  the  deeper  portions  of  the  ocean, 
how  is  it  possible  for  fish  and  other  animals  to  live  there? 

684.  What  is  the  ultimate  source  of  the  food  of  fishes  which  live 
near  the  surface  of  the  ocean,  in  the  open  sea? 

685.  Why  are  very  shallow  parts  of  the  ocean,  like  the  Grand  Banks 
of  Newfoundland  and  the  North  Sea,  such  great  fishing  grounds? 

686.  The  algal  flora  near  shore,  and  especially  near  mouths  of  large 
rivers,  is  apt  to  be  very  rich.     Explain. 

687.  What  great  element  of  the  food  supply  of  man  has  its  source  in 
the  algae? 

688.  Blue-green  algae  can  thrive  in  relatively  dry  places  as  compared 
with  other  algae.  What  structural  peculiarity  of  theirs  makes  this 
possible? 

689.  Blue-green  algae  are  generally  found  in  waters  which  are  rich 
in  organic  matter.  What  does  this  suggest  as  to  differences  between 
their  methods  of  nutrition  and  those  of  ordinary  green  plants? 

690.  Blue-green  algae  can  thrive  in  hot  springs  and  in  warmer  waters 
generally  than  can  most  of  the  other  algae.  Can  you  suggest  a  reason 
for  this? 

691.  What  do  we  mean  by  saying  that  one  group  of  plants  is  "on 
the  direct  line  of  ascent"  to  another? 

692.  Why  is  it  believed  that  the  green  algae  rather  than  the  other 
algal  gr()ui)s  are  on  the  direct  line  of  ascent  to  the  bryophytes? 

693.  What  arc  the  advantages  and  the  disadvantages  of  the  coeno- 
cytic  condition? 

694.  Brown  seaweeds  in  general  have  a  much  thicker  and  tougher 
plant  body  than  the  rest  of  the  algae.     Explain. 

695.  What  are  the  advantages  conferred  by  the  possession  of  bladder- 
like floats  in  the  brown  algae? 

696.  In  what  important  particular  is  fertilization  in  Fucus  different 
from  that  in  any  other  alga  described  in  the  text? 


THE  TlfALLOPHYTA  307 

697.  Which  of  the  other  algae  do  the  red  algae  most  resemble  in  their 
reproductive  structures? 

698.  \\'liat  makes  us  believe  that  the  fungi  have  come  from  the  algae 
rather  than  the  algae  from  the  fungi? 

699.  There  are  many  more  species  of  fungi  than  of  algae.     Explain. 

700.  Which  are  larger  plants,  on  the  whole,  the  fungi  ov  the  algae. 
Why? 

701.  Why  do  you  think  it  is  that  most  fungi  inhabit  the  land  and  most 
the  water? 


702.  Is  wind  or  water  the  commoner  agencj^  for  the  distribution  of 
the  spores  of  the  algae?  of  the  fungi? 

703.  AVhy  are  so  many  fungi  edible,  but  so  few  algae? 

704.  Why  are  fungi  so  much  more  important  economically  than  algae? 

705.  Through  what  steps  do  you  think  that  the  parasitic  habit  in 
fungi  may  have  arisen? 

706.  Spores  of  a  given  species  of  parasitic  fungus  will  generally 
germinate  and  grow  only  on  one  or  at  most  a  few  host  plants.  Is 
there  anything  analogous  to  this  "discriminating  power"  in  any  of  the 
other  physiological  activities  of  plants? 

707.  Why  are  the  fungous  diseases  of  plants  so  much  moi-e  common  in 
the  United  States  now  than  they  were  two  hundred  years  ago? 

708.  Why  is  a  wcnuid  much  more  lialjle  to  attack  by  fungi  than  is  a 
healthy  area? 

709.  Why  are  the  fungous  diseases  usuall}'  more  prevalent  in  wet 
seasons  than  in  dry? 

710.  Why  is  a  spray  of  such  a  substance  as  copper  sulphate  effective 
in  preventing  fungus  attacks  on  plants? 

711.  How  do  you  explain  the  fact  that  a  piece  of  moist  bread  shut 
up  in  a  box  will  almost  always  develop  mold? 

712.  Why  do  you  think  it  is  that  the  bacteria  are  the  commonest 
disease-producing  parasites  in  animals,  but  that  the  higher  fungi  are  the 
most  important  causes  of  disease  among  plants? 

713.  From  an  agricultural  point  of  view,  what  various  means  can 
you  suggest  for  combatting  the  attacks  of  the  fungous  diseases  of 
crop  plants? 


308  BOTANY:  PRINCIPLES  AND  PROBLEMS 

714.  What  evidence  is  there  that  bacteria  are  plants  rather  than 
animals? 

715.  The  minute  size  of  bacteria  is  thought  to  be  one  reason  why  they 
are  able  to  maintain  such  a  very  high  degree  of  physiological  activity. 
Why? 

716.  Why  cannot  plants  well  be  treated  with  serums  and  vaccines, 
as  can  animals? 

717.  In  killing  bacteria  in  liquids  by  heat,  it  has  been  found  that 
several  boilings,  each  followed  by  cooling  to  ordinary  temperature,  are 
much  more  effective  than  one  long  boiling.  Can  you  suggest  a  reason 
for  this? 

718.  Algae  themselves  are  not  harmful  in  drinking  water  but  their 
abundance  there  is  often  a  sign  that  such  water  may  be  dangerous 
to  drink.     Why? 

719.  Why  is  it  desirable  to  boil  surgical  instruments  before  using 
them  to  perform  an  operation? 

720.  Why  are  coughing  and  sneezing  particularl}'  dangerous  as 
means  of  spreading  infection? 

721.  Pasteur  found  that  a  sample  of  air  taken  in  Paris  contained 
many  bacteria  but  that  one  taken  in  the  high  Alps  was  free  from  these 
organisms.     Explain. 

722.  Can  you  think  of  any  reason,  aside  from  their  invigorating 
climate,  which  should  make  a  mountainous  or  arctic  region  particularly 
healthful? 

723.  Why  will  food  keep  longer  in  "cold  storage"  than  under  ordinary 
conditions? 

724.  Why  do  not  dried  vegetables  and  meats  decay? 

725.  Name  at  least  five  different  methods  which  are  employed  to 
preserve  food  and  keep  it  from  decaying,  and  state  what  makes  each 
method  effective. 

726.  What  group  of  algae  does  Rhizopus  most  resemble  in  its 
reproduction? 

727.  What  forms  among  the  algae  do  the  Mucorales  resemble  in 
having  plants  which  look  alike  but  are  entirely  different  sexually? 

728.  Some  kinds  of  fungi,  notable  the  truffles,  grow  only  around  cer- 
tain species  of  forest  trees.  What  explanation  for  this  fact  can  you 
suggest? 


THE  THALLOPHYTA  309 

729.  Teleutospores  are  thicker-walled   than  uredospores.      Explain. 

730.  Can  you  suggest  why  it  is  that  fleshy  fungi  are  often  so  good  to 
eat? 

731.  What  conditions  favor  a  luxuriant  growth  of  nuishrooms  antl 
toadstools? 

732.  Of  what  advantage  to  the  fungus  is  the  production  of  the 
"toadstool"  type  of  fruiting  body? 

733.  What  are  the  advantages  of  the  gills,  pores,  and  teeth  connnonly 
present  in  the  fruiting  bodies  of  the  fleshy  fungi? 

734.  What  is  the  advantage  of  the  "bracket"  form  of  fruiting  body? 

735.  Of  what  use  to  the  stink-horn  fungus  is  its  carrion-like  odor? 

736.  In  general,  what  insects  do  you  think  are  the  commonest  carriers 
of  fungus  spores?     Why? 

737.  In  a  lichen  plant,  what  is  the  advantage  gained  by  the  fungus 
and  what  by  the  alga? 

738.  "Slavery"  is  the  term  sometimes  used  to  describe  the  relation 
of  alga  to  fungus  in  the  lichen  plant.  Why  is  "slavery  "  perhaps  a  better 
term  than  "symbiosis"  or  "parasitism?" 

739.  How  different  in  general  is  the  substratum  on  which  lichens 
grow  from  that  which  supports  algae  or  fungi? 

740.  Lichens  particularly  like  rough  surfaces  to  grow  on.     Explain. 

741.  Since  algae  and  fungi  are  both  usually  best  developed  in  a  moist 
environment,  how  does  it  happen  that  lichens  will  often  thrive  in  dry 
situations? 

742.  Lichens  are  greener  when  moist  than  when  dry.     Explain. 

743.  The  vegetative  body  of  a  lichen  is  more  thin  and  thallus-like 
than  that  of  most  fungi.     Explain. 

744.  Aside  from  their  small  size,  why  do  you  think  it  is  that  the  algal 
members  of  most  lichens  are  either  Cyanophyceae  or  Protococcales? 

745.  What  conditions  must  be  present  before  an  ascospore  produced 
by  a  lichen  plant  can  give  rise  to  a  new  lichen  plant? 

746.  Of  what  importance  are  lichens  in  nature? 

747.  What  other  instances  do  you  know  of,  aside  from  lichens,  where 
fungi  become  intimately  associated  with  green  jjlants? 


310  BOTANY:  PRINCIPLES  AND  PROBLEMS 

REFERENCE  PROBLEMS 

117.  Name  some  algae  which  are  of  economic  importance. 

118.  What  is  "diatomaceous  earth"  and  how  was  it  produced? 

119.  Give  an  example  of  a  change  in  some  agricultural  practice  which  has 
come  about  since  the  existence  and  importance  of  bacteria  have  become 
recognized. 

120.  What  is  "Pasteurization"  and  how  is  it  useful? 

121.  Species  of  bacteria  which  look  alike  may  often  be  distinguished 
from  one  another  by  differences  in  their  physiological  activity.  What 
examples  can  you  cite  from  the  seed  plants  of  the  use  of  physiological 
differences  to   distinguish  two  plant  varieties  which  look  very  similar? 

122.  What  is  the  life  history  of  the  white  pine  blister  rust  and  what  is  a 
good  means  for  combatting  this  fungus? 

123.  What  are  fungus  galls  and  how  are  they  produced? 

124.  What  are  "fairy  rings"  and  how  do  you  explain  them? 

125.  How  are  truffles  gathered? 

126.  What  parasitic  animals  are  there  whose  life  history  is  somewhat 
analogous  to  that  of  the  rusts? 

127.  Who  first  discovered  that  lichens  are  composed  of  algae  and 
fungi  ? 

128.  What  lichens  are  of  economic  importance? 

129.  Give  the  derivations  of  the  following  terms  and  explain  why  each  is 
appropriate : 

Heterocyst  Pyrenoid  Basidium 

Zoospore  Plankton  Perithecium 

Zygospore  Trichogyne  Hymenium 

Oospore  Mycelium  Sterigma 

Heterogamy  Toxin  Teleutospore 

Antheridium  Conidium  Uredospore 

Oogonium  Aseus  Aecidium 
Soredium 


CHAPTER  XV 
THE  BRYOPHYTA 

The  second  of  the  four  great  divisions  of  the  plant  kingdom 
is  the  Bryophyta  or  bryophytes,  the  members  of  which  we  know 
commonly  as  the  liverworts  and  mosses.  This  group  is  much 
smaller  than  the  thallophytes,  containing  only  about  16,000  species, 
nor  does  it  approach  them  in  the  diversity  of  plant  types  which  it 
displays.  All  of  its  members  are  lowly  and  inconspicuous,  the 
tallest  mosses  rarely  attaining  a  decimeter  in  height.  From  the 
economic  point  of  view  the  group  is  of  very  minor  consequence. 
To  the  botanist,  however,  the  bryophytes  are  of  particular 
interest  in  helping  to  picture  for  us  those  ancient  plants  which 
first  crept  out  of  the  sea  to  invade  the  dry  land,  and  which 
therefore  took  the  first  steps  along  the  evolutionary  road  leading 
up  to  our  dominant  and  familiar  seed  plants. 

The  bryophytes  are  undoubtedly  a  very  ancient  group  and 
their  history  is  necessarily  obscure.  We  have  good  reason 
to  believe,  however,  that  they  arose  from  plants  resembling 
some  of  our  higher  algae  of  today,  and  several  connecting  links 
between  algae  and  liverworts  have  accordingly  been  suggested. 
The  two  main  characteristics  which  distinguish  the  bryophytes 
as  a  whole  from  these  ancestral  thallophytes  are  the  establish- 
ment of  a  clearly  marked  alternation  of  generations  and  the 
possession  of  multicellular  sexual  organs. 

Alternation  of  Generations. — As  outlined  in  a  previous  chapter, 
these  plants  possess  a  definite  sexual  gamete-producing  member, 
the  gametuphyte,  which  is  followed  in  the  life  history  by  an  equally 
definite  non-sexual,  spore-producing  member,  the  sporophyte. 
The  sporophyte  here  is  little  more  than  a  spore  case  and  is  always 
attached  to  the  tissues  of  the  gametophyte,  never  becoming  an  in- 
dependent individual  as  it  does  among  the  ferns  and  their  allies. 
The  V)eginnings  of  this  alternation  of  generations,  as  we  have  seen, 
make  their  appearance  here  and  there  among  the  thallophytes, 
but  when  we  reach  the  liverworts  and  mosses  it  becomes 
regularly  established  and  is  henceforth  a  distinctive  feature  in 
the  life  histories  of  all  species  throughout  the  plant  kingdom. 

311 


312 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Multicellular  Sexual  Organs. — The  sexual  organs  of  the 
bryophytes  have  also  attained  a  degree  of  complexity  far  above 
those  of  the  thallophytes.  In  the  latter  group,  with  a  few  minor 
exceptions,  the  structures  which  produce  the  eggs  and  the  sperm 
are  modified  single  cells,  the  gametes  being  formed  directly  out  of 
the  cell  contents.  In  the  bryophytes,  however,  the  gamete- 
producing  organ  has  a  definite,  many-celled  wall  surrounding  the 


Fig.  183. — Multicellular  sexual  organs  of  a  bryophyte  (Ricciocarpus) .  A, 
antheridium.  B,  archegonium.  v,  venter,  n.  neck.  A  large  egg  is  evident  in 
the  venter,  but  the  ventral  canal  cell  and  neck-canal  cells  are  largely  broken 
down,  following  the  opening  of  the  neck  of  the  archegonium.  See  also  Figs.  185 
and  196. 


cell  or  group  of  cells  which  develop  into  the  gametes  (Fig.  183). 
The  female  sex  organ  is  now  known  as  the  archegonium.  It  is  a 
somewhat  flask-shaped  structure,  the  swollen  lower  portion  of 
which  is  known  as  the  venter  and  the  elongated  upper  portion  as 
the  neck.  The  wall  is  a  single  cell-layer  in  thickness.  Most  of 
the  cavity  of  the  venter  is  occupied  by  a  large  egg-cell,  and  just 
above  this  lies  a  much  smaller  ventral  canal  cell.  Filling  the  neck 
are  a  row  of  narrow  neck-canal  cells.  When  wet,  the  neck  opens, 
the  neck-canal  cells  breakdown,  and  a  sperm  enters  to  fertilize  the 
egg.  The  male  sex  organ  is  still  known  as  the  antheridium.  In 
bryophytes  it  has  typically  a  short  stalk  and  is  somewhat  elon- 
gated. Its  wall,  one  cell-layer  in  thickness,  surrounds  a  mass  of 
small,  squarish  cells  within  each  of  which  a  motile  sperm  is 
developed,  provided  with  two  cilia.     When  wet,  the  antheridium 


THE  HRYOPIIYTA  313 

breaks  open  and  libeiates  the  sperms,  which  swim  about  and 
under  favorable  conditions  enter  archegonia  and  effect  fcrtihza- 
tion.  Archegonia  and  antheridia,  or  structures  which  have  been 
derived  from  them,  may  be  recognized  in  the  gametophytes  of  all 
the  remaining  members  of  the  plant  kingdom. 

Bryophytes  are  divided  into  two  classes,  the  Ih'paticae  and 
the  Musci. 

Hepaticae  or  Liverworts. — The  members  of  this  class  arc  low- 
growing  plants,  chiefly  inhabiting  moist  places.     Their  vegeta- 


0^1{^^6 


Via.  184. — Ricciocarpus,  a  simple,  floating  liverwort.  The  round  bodies 
lying  at  the  bottoms  of  the  furrows  on  its  surface  are  sporophytes,  each  of  which 
has  arisen  there  from  a  fertilized  egg  in  an  archegonium.  On  the  lower  surface  of 
the  thallus  are  groups  of  thread-like  absorbing  organs  or  rhizoids. 

tive  body  is  a  flattish  thallus  which  creeps  over  the  surface  of 
the  ground,  and  is  attached  thereto  by  thread-like  filaments  or 
rhizoids.  In  some  types  it  is  cut  up  into  leaf-like  lobes.  Here  are 
found  the  lowest  and  most  alga-like  of  the  bryophytes.  They  are 
commonly  divided  into  the  M ar chant i ales,  Jungennanniales,  and 
Anthocerotales. 

1.  Marchantiales. — The  thallus  of  these  plants  is  a  thick, 
dichotomously  forking  structure,  in  the  upper  or  dorsal  portion 
of  which  occur  air  spaces  or  chambers,  communicating  directly 
with  the  outside  and  thus  freely  exposing  to  the  air  the  delicate 
chlorophyll-bearing  cells  of  the  interior.  In  Riccia  and  its  allies 
(Fig.  184),  the  simplest  members  of  the  order,  the  archegonia 
and  antheridia  (Figs.  183  and  185)  occur  along  grooves  in  the 
upper  portion  of  the  thallus  and  are  sunken  just  below  its  surface. 
The  sporophyte  (Fig.  185)  which  develops  from  the  fertilized  egg 
is  nothing  more  than  a  spore  case  or  sporangium,  often  called 
among  bryophytes  the  sporogoniuni,  and  here  consists  merely  of  a 
mass  of  spores  surrounded  by  a  definite  wall.  These  spores,  like 
all  produced  by  sporophytes,  occur  in  tetrads  or  groups  of  four. 


314 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


The  sporogonium  is  sunken  in  the  tissues  of  the  thalkis,  to  which 
it  is  attached  at  the  base  and  from  which  it  draws  its  food.  In 
the  higher  meml^ers  of  the  order,  as  ilhistrated  by  the  common 


Fig.  185. — Ricciocarpus  natans.  A,  archegonium.  X  200.  B,  antheridium. 
X  150.  C,  sporophyte,  contained  within  the  remains  of  the  archegonium,  and 
with  a  thin  wall  of  its  own.      The  spores  are  borne  in  groups  of  four.       X  100. 


Fig.  186. — Marchantia.     Thallus  bearing  female  receptacles,  in  which  occur 
the  archegonia.     A  single  male  or  antheridial  receptacle  is  shown  separately. 


genus  Marchantia  (Fig.  186),  the  sexual  organs  are  borne  on 
speciahzed  discs,  each  carried  up  above  the  surface  of  the  thallus 
by  a  stalk.     The  sexes  are  separate  here,  some  gametophytes 


THE  BRYOI'HYTA 


315 


Marchantia.     Section    through    the    male    receptacle,    showing  thi 
antheridia  sunken  below  the  surface. 


188.  1  IG     ISM 

Fig.  188. — Marchantia.  Section  through  female  recoptaelc.  a,  archogonia. 
two  at  left  unfertilized,  two  at  right  with  young  sporophytes.  m,  membranous 
fringe.     I,  finger-like  lobe. 

Fig.  189. — Marchantia.  Sporophyte,  developed  from  a  fertilized  egg.  The 
enlarged  capsule  contains  spores  and  elaters.  It  is  supported  by  a  stalk  or  seta, 
which  is  embedded  in  the  tissues  of  the  gametophyte  by  a  foot.      X  4.5. 


316  BOTANY:  PRINCIPLES  AND  PROBLEMS 

bearing  only  anthcridia  (Fig.  187)  and  some  only  archegonia 
(Fig.  188).  The  sporophyte  (Fig.  189)  is  more  specialized  than 
that  of  Riccia  for  it  includes  not  only  a  spore  case  but  a  short 
stalk  or  seta  the  growth  of  which  carries  the  spore  case  out  and 
away  from  the  thallus.  The  base  of  this  seta  is  enlarged  into  a 
foot,  anchoring  the  sporophyte  in  the  tissue  of  the  thallus  and 
absorbing  food  therefrom.  Furthermore,  not  all  of  the  central 
portion  of  the  sporogonium  itself  produces  spores,  for  many  of  the 


Fig.   190. — Porella.     Portion  of  thallus,  showing  the  two  rows  of  leaves.     One  of 
the  two  capsules  figured  has  opened  and  liberated  its  spores. 

cells  grow  instead  into  long,  spirally  thickened  elements,  the  elaters, 
which  assist  in  loosening  and  scattering  the  spore  mass  at  maturity. 
The  relative  amount  of  sporophyte  tissue  which  does  not  contrib- 
ute directly  to  the  production  of  spores,  and  is  therefore  called 
sterile  tissue,  increases  steadily  as  we  trace  the  upward  evolution 
of  the  sporophyte.  In  the  simplest  case  (among  certain  thallo- 
phytes),  the  fertilized  egg  develops  into  a  group  of  spores.  In 
Riccia,  the  only  sterile  tissue  is  the  sporangium  wall;  in  Marchan- 
tia,  the  seta  and  foot  are  added;  in  the  mosses  still  other  regions 
are    "sterilized",    and    in   the   higher   plants   the   spores  them- 


TIIK  BRYOP/IVTA  317 

selves  constitute^  Init  a  vcny  small  portion  of  the  sporophytc  asa 
whole, 

2.  Jungermatinhiles  or  Leafy  Liverworts. — In  number  of 
species  this  order  is  by  far  the  largest  of  the  three  groups  of 
liverworts.  Its  thallus  is  much  less  complex  internally  than 
that  of  the  Marchantiales.  Externally,  however,  it  is  more 
specialized,  for  in  most  species  it  is  divided  into  a  slender  axis 


Fig.   191. — Anthoceros.     Portion  of  thallus,  showing  several  sporophytes,  one  of 
which  is  splitting  open  and  liberating  the  spores. 

or  stem  and  three  crowded  rows  of  small  and  delicate  lobes 
or  "leaves"  (Fig.  190).  The  stem  never  rises  to  an  erect  position 
but  is  always  prostrate  on  the  ground,  to  which  it  is  attached  by 
rhizoids.  Sex  organs  are  borne  on  the  main  axis  or  on  short 
lateral  branches.  The  sporophyte  develops  a  much  longer 
seta  than  in  the  Marchantiales  and  the  spore  case  contains  still 
more  sterile  tissue.  At  maturity  it  breaks  open  into  four  spread- 
ing lobes. 

3.  Anthocerotales. — The  gametophyte  in  this  grouji  is  a  simple, 
flat  thallus  (Fig.  191),  but  the  sporophyte  is  remarkable  in  several 
particulars.  In  the  genus  Anthoceros,  the  best  known  member  of 
the  order,  the  sporophyte  (Fig.  192)  is  long  and  slender  and  is 


318 


BOTAXY:  PRINCIPLES  AND  PROBLEMS 


well  anchored  in  the  thallus  by  its  foot.  Just  above  the  foot  is 
a  growing  region,  through  the  activity  of  which  the  sporophyte 
continues  to  elongate  during  the  whole  season.     The  spores  at 


Fig.  192. — The  sporophyte  of  Anthoceros.  Longitudinal  section  of  basal 
portion,  showing  the  large  foot,  embedded  in  the  gametophyte.  The  spore-bear- 
ing tissue  surrounds  the  sterile  columella  within,  and  is  surrounded  by  a  layer  of 
sterile,  chlorophyll-bearing  tissue  without.  The  base  of  the  sporophyte  con- 
tinues to  grow,  adding  progressively  to  these  various  tissues. 


the  tip  ripen  first,  and  ripening  proceeds  slowly  downward,  the 
spore-case  gradually  splitting  into  halves  (Fig.  191).     Its  internal 


TIIK  HRYOl'IIYTA  319 

structure  is  more  highly  differentiated  than  in  any  other  of  the 
bryophytes.  A  core  of  sterile  tissue,  the  columella,  occupies 
the  center  or  axis.  Around  this  is  a  layer  of  spores,  which  in 
many  species  is  broken  up  by  groups  of  sterile  cells.  The  wall 
outside  this  is  three  or  four  cell  layers  in  thickness,  and  except 
for  the  outermost  one,  or  epidermis,  its  cells  are  provided  with 
chlorophyll  and  are  often  separated  somewhat  by  intercellular 
air  spaces.  Well  developed  stomata,  with  guard  cells  much  like 
those  in  the  higher  plants,  occur  in  the  epidermis.  The  sporo- 
phyte  of  the  Anthocerotales  is  therefore  able  to  carry  on  photo- 
synthesis actively,  though  it  still  necessarily  depends  upon  the 
gametophyte  for  water  and  mineral  salts.  This  group  has  always 
been  of  particular  interest  to  botanists  as  suggesting  a  possible 
connection  between  the  bryophytes  and  those  higher  plants  in 
which  the  sporophyte  is  an  independent  individual. 

Musci  or  Mosses. — The  mosses  are  much  commoner  and  more 
familiar  plants  than  the  liverworts,  and  under  certain  conditions 
form  an  important  element  in  the  vegetation.  Many  of  them 
thrive  only  in  moist  situations  but  others  are  common  on  ordinary 
dry  soil  and  still  others  live  under  exceptionally  xerophytic 
conditions  where  few  plants  can  grow.  The  moss-plant  is 
typically  erect  and  consists  of  a  stalk  around  which  small,  delicate 
leaves  are  arranged  in  spirals.  The  stem  has  very  little  internal 
differentiation  and  the  leaves  are  only  one  or  two  cells  in  thick- 
ness, so  that  the  vegetative  organs  are  far  from  approaching  in 
complexity  those  of  ferns  and  seed  plants.  As  in  the  liverworts, 
the  plant  is  attached  to  the  soil  by  thread-like  rhizoids.  The 
sporophyte  also  shows  an  advance  over  earlier  conditions  in  a 
progressive  increase  in  sterile  tissue,  particularly  in  the  higher 
forms;  and  in  opening  by  a  distinct  lid,  or  operculum,  at  the  top. 

Two  main  orders  are  recognized,  the  Sphognales  and  the 
Bryales. 

1.  Sphagnales  or  Peat  Mosses. — These  all  belong  to  the  single 
large  genus  Sphagnum,  characteristic  of  the  bogs  and  swampy 
regions  of  temperate  climates.  The  spore  germinates  into  a 
flat,  thallus-likc  structure  from  the  surface  of  which  arise  upright 
and  much-branched  shoots,  thickly  covered  with  small  leaves 
(Fig,  193).  Many  of  the  leaf  cells  are  dead  and  empty  and  are 
so  constructed  that  they  will  absorb  and  hold  large  quantities  of 
water.  At  the  tips  of  the  main  branches  are  borne  the  sexual 
organs.     The  globular  capsule  is  provided  with  a  well-devclopiMl 


320 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


foot,  its  wall  is  thick,  and  much  of  the  central  tissue  is  sterile, 
the  spores  occurringjn  a  rather  small,  dome-shaped  mass  in  the 
upper  portion.  There  is  no  true  seta  but 
the  sporophyte  is  carried  upward  on  a  stalk 
formed  by  the  gametophyte. 

2.  Bryales  or  True  Mosses. — These  are 
the  common  mosses,  numbering  over  12,000 
species  and  widely  distributed  over  the 
globe.  Many  of  them  are  particularly  well 
suited  to  live  in  cold  or  dry  situations  and 
they  often  form  the  vanguard  of  an  ad- 
vancing vegetation. 

The  spore  does  not  germinate  directly 
into  what  we  know  as  a  moss  plant,  but 
produces  instead  a  mass  of  green  filaments, 
the  protonema  (Fig.  194).  From  this  arise 
erect  branches  which  grow  into  the  leafy 
shoots  with  which  we  are  familiar  (Figs. 
(i  and  195).  The  stem  is  usually  but  a  few 
centimeters  in  height  and  shows  little  com- 
plexity, although  there  may  often  be  dis- 
tinguished a  firm  central  mass  of  tissue — 
presumably  the  region  of  conduction — and 
a  softer  zone  outside.  Nothing  approaching 
the  highly  differentiated  stem  structure  of  ferns  and  seed  plants 


Fig.  WS.^ Sphagnum. 
Leafy  shoot  of  the  game- 
tophyte, with  three  cap- 
sules at  the  top. 


Fig.  194. — Moss  protonema,  the  deHcate,  thread-Hke  structure  which  grows 
from  the  germinated  moss  spore.  Along  this  protonema  several  young  moss 
plants  are  arising,  the  one  at  the  left  well  started,  the  other  three  mere  buds. 

is  present,  however.     The  leaves  are  typically  small  and  narrow, 
and  but  one  layer  of  cells  in  thickness.     They  may  often  become 


THE  IWYOPIIYTA 


321 


very  dry  and  still  retain  their  vitality.  The  filaments  of  pro- 
tonema,  to  which  the  moss  plant  still  remains  attached,  continue 
to  grow  and  serve  as  a  means  for  anchorage,  absorption,  and 
dispersal. 


ABC 

Fig.  195.  Fig.  196. 

Fig.  195. — A  moss  (,Polylricfmmco?)i in tiiir,  the  huir-cup  moss),  yl,  male  plant 
wliich  bears  antheridia  at  its  tip.  B,  female  plant,  showing  mature  sporophyte. 
C,  female  plant  with  its  various  portions  separated,  g,  gametophyte  or  moss 
plant,  s,  seta  or  stalk,  c,  capsule,  o,  operculum,  a,  calyptra.  (From 
Gager's  "Fundamentals  of  Botany",  P.  Blakiston's  Son  and  Co.,  Philadelphia). 

Fig.  196. — Sexual  organs  of  a  moss  {Mnium).  A,  archegonium  (in  sec- 
tion).    B,  antheridium.     X  100. 

Sexual  organs  (Fig.  196)  are  borne  at  the  tips  of  the  branches, 
sometimes  on  the  same  plant  but  often  on  separate  individuals. 
The  sporophyte  usually  develops  a  long  seta,  the  growth  of  which 
carries  the  capsule  far  up  above  the  moss  plant.     Remains  of 


322 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


the  archegonium  are  carried  up  with  it  and  form  a  protecting 
cap  or  calypira  which  covers  the  young  capsule.  The  capsule 
itself  (Fig.  197)  possesses  a  central  columella  and  much  other 
sterile  tissue,  so  that  the  spore-bearing  layer,  which  surrounds 
the  columella,  is  relatively  thin.     The  operculum  drops  off  at 


Fig.  197. — Longitudinial  section  through  the  capsule  of  a  moss  (Polytrichum). 
st,  seta,  ap,  spophysis.  w,  wall,  c,  columella,  s,  spore-sac,  surrounded  on 
both  sides  by  loose  tissue.     {From  Goebel). 

maturity  and  the  liberation  of  the  spores  is  controlled  by  a  ring 
of  teeth  projecting  inward  from  the  edge  of  the  wall.  Just 
below  the  capsule  there  is  typically  an  enlarged  region,  the 
apophysis,  which  is  often  brightly  colored  and  may  sometimes 
possess  chlorophyll.  The  capsule  is  very  diverse  in  size,  shape, 
and  structure  throughout  the  whole  group. 


THE  Bin  OP/I  VTA  323 

Fertilization  is  probably  offocted  rather  rarely,  owing  to  the 
fact  that  an  abundance  of  water  is  necessary  in  which  the  sperms 
may  swim  about.  Perhaps  as  a  result  of  this  we  find  many 
devices  among  the  mosses  by  which  asexual  reproduction  is 
effected,  and  most  of  the  new  individuals  produced  probably 
arise  by  this  means. 

Despite  its  large  size  and  relatively  high  specialization,  the 
mosses  are  not  believed  to  have  led  directly  to  any  of  the  higher 
plant  groups,  but  we  look  instead  to  such  simpler  forms  as 
Anthoceros  for  a  suggestion  as  to  how  the  great  gap  which  now 
exists  between  bryophytes  and  pteridophytes  may  have  been 
bridged. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

748.  What  great  change  in  the  character  of  the  spores  occurs  as  we 
pass  from  algae  to  bryophytes? 

749.  What  is  the  advantage  of  a  multicellular  sexual  organ  over  the 
primitive  unicellular  type? 

750.  What  reason  can  you  suggest  for  the  presence  of  the  ventral 
canal  cells  and  the  neck-canal  cells  in  the  archegonium? 

751.  What  exceptions  are  there  to  the  general  rule  that  the  thallo- 
phytes  have  unicellular  sexual  organs? 

752.  Where  else  in  the  plant  kingdom  below  the  liverworts  have  we 
found  dichotomous  branching?  Do  you  think  that  it  is  primitive  or 
not  ?     How  does  it  differ  from  the  typical  branching  of  the  higher  plants  ? 

753.  Of  what  advantage  to  the  moss  plant  is  the  seta? 

754.  Why  do  you  think  it  is  that  the  bryophytes  have  never  been 
able  to  produce  plants  of  any  great  height? 

755.  In  bryophytes,  what  are  the  advantages  and  the  disadvantages 
of  the  thallus  type  and  of  the  leafy  type  of  plant? 

756.  In  the  general  character  of  their  thallus,  what  group  of  algae 
do  the  liverworts  most  resemble? 

757.  Why  do  you  think  it  is  that  the  liverworts  are  largely  eoufined 
to  moist  places,  while  the  mosses  often  thrive  in  relatively  dry  ones? 

758.  AMiat  notable  resemblance  is  there  in  structure  between  the 
thallus  of  the  liverworts  and  the  leaves  of  the  higher  plants? 


324  BOTANY:  PRINCIPLES  AND  PROBLEMS 

759.  What  are  the  advantages  of  the  system  of  air  chambers  in  the 
thallus  of  the  Marchantiales? 

760.  Enumerate  the  various  ways  in  which  the  Anthocerotales  are 
closer  to  the  higher  plants  than  are  the  rest  of  the  bryophytes. 

761.  Name  all  the  ways  you  can  think  of  in  which  the  sporophyte  of 
Anthoceros  resembles,  and  differs  from,  that  of  the  seed  plants. 

762.  The  shoots  of  Sphagnum  are  greenish  or  grayish-green  instead 
of  dark  green  as  in  most  mosses.     Why? 

763.  Why  is  sphagnum  (the  dead  and  dried  remains  of  Sphagnum 
plants)  a  very  good  absorbent? 

764.  What  important  part  in  the  economy  of  nature  is  played  by 
the  mosses? 

765.  What  does  a  moss  protonema  resemble,  and  what  does  its  i)res- 
ence  suggest  as  to  the  ancestry  of  the  mosses? 

766.  Bryophytes    are    generally   very   tolerant   of  shade.     Explain. 

767.  Where  at  present  on  the  earth  is  vegetation  "headed  by  a 
vanguard  of  mosses"? 

768.  From  your  study  of  the  bryophytes  (and  of  other  plant  groups) 
do  you  think  that  all  parts  of  the  plant  change  at  the  same  rate  during 
the  progress  of  evolution?     Explain  and  illustrate. 

REFERENCE  PROBLEMS 

130.  Where  does  peat  come  from  and  how  has  it  been  formed? 

131.  Why  is  sphagnum  so  much  used  for  surgical  dressings? 

132.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Archegonium  Columella  Hypophysis 

Elater  Operculum  Protonema 

Calyptra 


CHAPTER  X\T 
THE  PTERIDOPHYTA 

In  passing  from  the  bryophytes  to  the  pteridophytes,  which 
inchide  the  ferns,  club  mosses,  and  horsetails,  we  cross  the  widest 
gap  which  exists  in  the  continuity  of  the  plant  kingdom.  Inter- 
mediate forms  between  the  liverworts  and  mosses  on  the  one  hand 
and  the  ferns  and  their  allies  on  the  other  are  missing,  and 
although  we  may  suggest  various  connecting  links  and  reconstruct 
plausible  evolutionary  series,  the  transitional  plants  themselves 
have  long  since  perished  and  we  shall  probably  never  know  just 
how  our  present  typical  land  vegetation  had  its  origin. 

The  Advance  from  Bryophytes  to  Pteridophytes. — In  the 
advance  from  bryophytes  to  pteridophytes  the  relative  importance 
of  the  two  generations  has  been  completely  reversed.  The 
sporophyte  is  no  longer  an  appendage  of  the  gametophyte  but  is 
now  the  dominant  and  conspicuous  generation,  and  has  attained 
complete  independence.  The  sexual  plant  is  still  independent, 
too,  but  it  is  relatively  small  and  insignificant,  and  from  this 
point  onward  throughout  the  vegetable  kingdom  it  suffers  a 
steady  and  progressive  reduction.  This  shift  in  evolutionary 
advance  from  the  gametophyte  to  the  sporophyte  marks  the 
completion  of  that  great  forward  step  in  the  plant  kingdom 
whereby  a  true  land  vegetation  was  evolved.  The  gametophyte 
is  primarily  an  aquatic  or  at  least  a  moisture-loving  structure. 
Even  in  its  highest  development  among  the  mosses,  where  it 
successfully  invades  the  dry  land,  it  has  never  been  able  to  produce 
there  a  strong  and  vigorous  vegetation.  The  sporophji^e, 
however,  seems  early  to  have  solved  the  problem  presented  by 
this  radical  change  in  environment,  and  when  we  meet  it  in  the 
pteridophytes  it  has  already  developed  a  stout,  branching,  sub- 
terranean axis,  the  root,  clothed  with  an  abundance  of  root-hairs 
for  absorption;  large  leaves  presenting  to  the  sun  a  relatively 
thick  layer  of  chlorophyll-bearing  tissue,  which  is  well  provided 
with  air  spaces  and  is  protected  by  an  epidermis  in  which  are 
typical  stomata;  and  a  stout  stem  on  which  the  leaves  are  lifted 

325 


326  BOTANY:  PRINCIPLES  AND  PROBLEMS 

high  in  air  and  which  has  made  possible  the  development  of  the 
tall  and  vigorous  plant  body  with  which  we  are  familiar.  This 
advance  in  external  complexity  is  paralleled  by  an  equally 
notable  internal  one,  for  instead  of  the  relatively  simple  structure 
of  the  moss  plant,  we  find  the  highly  differentiated  internal 
anatomy  described  in  earlier  chapters.  This  is  chiefly  distin- 
guished by  the  development  of  those  tissues  for  support  and 
conduction  which  we  call  fibro-vascular,  and  which  include  the 
wood  and  the  bast.  So  distinctive  of  pteridophytes  and  seed 
plants  is  this  type  of  internal  structure  that  these  groups  are 
sometimes  known  collectively  as  the  vascular  plants,  in  distinction 
from  the  non-vascular  thallophytes  and  bryophytes. 

The  remarkable  advance  in  vegetative  structures  which  the 
pteridophytes  display  is  not  paralleled  in  their  methods  of 
reproduction.  The  gametophytes  form  archegonia  and  antheri- 
dia,  though  somewhat  smaller  and  simpler  ones  than  those  of  the 
mosses,  and  motile  sperms  swim  to  the  archegonia  and  there 
effect  fertilization.  The  amount  of  sterile  tissue  in  the  sporo- 
phyte  has,  of  course,  enormously  increased,  but  typical  spores  are 
still  produced  in  definite  sporangia  and  scattered  abroad  just 
as  they  are  among  the  mosses.  In  the  higher  members  of  the 
division,  two  kinds  of  spores  appear:  Microspores,  which  give  rise 
to  antheridium-producing  or  male  gametophytes,  and  megaspores, 
which  give  rise  to  archegonium-producing  or  female  gameto- 
phytes. This  condition  of  heterospory  foreshadows  the  evolu- 
tion of  the  seed,  which  distinguishes  the  last  and  highest  plant 
group,  the  seed  plants. 

Pteridophytes  are  not  very  numerous  in  species  nor  do  they 
form  a  very  conspicuous  part  of  the  earth's  vegetation  today 
except  in  certain  moist  and  warm  regions.  A  study  of  fossil 
plants,  however,  shows  us  that  members  of  this  division  were 
much  more  common  in  past  ages,  and  indeed  at  certain  periods 
were  the  most  notable  element  in  the  plant  population.  More- 
over, at  that  time  they  included  many  stout,  woody,  tree-like 
species  which  formed  great  forests.  In  competition  with  seed 
plants,  the  group  soon  fell  from  its  dominant  position  and  the  few 
descendants  which  it  has  left  to  the  present  day  are  for  the 
most  part  reduced  and  degenerate. 

Three  classes  are  recognized  among  the  existing  pteridophytes; 
the  Filicineae  or  Ferns,  the  Lycopodineae  or  Club  Mosses  and  the 
Equisetineae   or   Horsetails.     These   are  so  different  from  one 


77//';  rTKRIDOPHYTA 


32i 


another  that  they  are  sometimes  regarded  as  three  distinct  divi- 
sions, but  their  many  points  of  resemblance  make  it  perhaps  more 
satisfactory^  to  group  them  together. 

Filicineae  or  Ferns. — ^This  class,  the  largest  of  the  three, 
includes  the  most  conspicuous  and  familiar  of  the  pteridophytes. 
The  leaves,  here  known  as  fronds,  are  typically  large  and  are 


Fig.  198. — A  fern  plant,  the  polypody  {Polypodium  vulgare).  This  is  the 
sporophyte  generation.  The  stem  is  a  creeping  rootstock.  On  the  backs  of  the 
leaves  are  born  sori,  or  clusters  of  sporangia. 


often  deeply  cut  and  dissected.  The  spores  are  all  alike, 
except  in  the  small  group  of  water-ferns  where  heterospory  exists. 
With  a  few  exceptions,  the  gametophyte  grows  on  the  surface  of 
the  soil  and  is  provided  with  chlorophyll,  thus  existing  as  an 
entirely  independent  plant.  Three  orders  are  recognized,  the 
Filicales,  Ophioglossales,  and  Hydropteridales. 

1.  Filicales  or  True  Ferns  (Figs.  198  and  199).— Almost  all  of 
the  ferns  belong  here,  the  other  two  orders  being  very  small 
ones.     In  our  common  species  the  stem  is  much  reduced  and  is 


328 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fig.  199. — The  structure  of  a  fern  (Aspidium).  A,  the.  plant  as  a  whole. 
B,  portion  of  leaf  with  seven  fruiting  dots  or  sori  on  its  lower  surface.  Each  is 
covered  by  an  indusium  (a),  from  under  which  the  sporangia  (6)  are  protruding, 
in  one  case.  C,  a  single  sporangium.  D,  transverse  section  through  a  sorus, 
showing  section  of  leaf-blade  above  and  of  indusium  below,  with  cluster  of 
sporangia  attached  between  them.      {From  Strasburger,  after  Wossidlo). 


THE  PTERIDOPII  VTA 


329 


prostrate  or  subterranean,  so  that  the  leaves  ai)i)ear  to  rise  directly 
from  the  ground.  In  many  tropical  species,  however,  an  erect 
trunk  is  produced  which  ])ears  a  crown  of  large  leaves  at  its 


Fig.  200. — Transverse  section  (diagrammatic)  of  the  stem  of  a  fern  {Adian- 
tum).  The  fibrovascular  system  is  here  arranged  in  a  hollow  tube,  the  wood 
surrounded  by  bast  both  within  and  without.  At  the  right,  a  segment  of  the 
cylinder  is  passing  off  to  a  leaf  as  a  leaf-trace,  causing  a  temporary  break,  or  leaf- 
gap,  in  the  cylinder.     Wood  black,  bast  dotted. 

summit.  The  fern  stem  lacks  a  cambium,  and  the  secondary 
wood  and  bast  so  common  in  the  seed  plants  are  consequently 
absent.     The  fibro-vascular  system  may  occasionally  be  a  solid 


Fig.  201. — Fern  sporangia  of  various  types.  The  ring  of  heavy-walled  cells  is 
the  annulus,  by  the  contraction  of  which  the  wall  of  the  sporangium  is  ruptured 
at  maturity  and  its  spores  scattered.     {From  Strasburgcr) . 


axis,  but  is  connnonly  a  i-ing  or  tube  surrountling  a  central  pith 
(Fig.  200)  and  is  often  broken  up  by  gaps  into  a  series  of  sei)arate 
bundles.     The  bast  here  occurs  not  only  on  1h(>  outside  of  1h(> 


330 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


wood  but  also  inside,  next  the  pith,  and  thus  completely  surrounds 
the  wood.     The  cells  of  both  wood  and  bast  are  somewhat  less 


Fig.  202. — The  gametophyte  of  a  fern.  View  of  the  under  surface,  which  lies 
next  the  surface  of  the  ground.  Here  are  borne  the  archegonia  (near  the  notch) 
and  the  antheridia  (farther  back,  among  the  rhizoids). 


Fig.  203  — Sexual  organs  of  a  fern.  A,  section  of  archegonium  just  before 
maturity.  Within  can  be  distinguished  the  large  egg  cell,  below  which  are  the 
ventral  canal  cell  and  two  neck-canal  cells.  In  B,  the  archegonium  is  mature  and 
its  neck  has  opened.  The  egg  and  ventral  canal  cells  are  evident,  but  the  neck- 
canal  cells  have  broken  down.  C,  section  of  antheridium,  showing  basal  cell  (1), 
ring  cell  (2),  and  cap  cell  (3).     D,  one  of  the  sperms,  more  highly  enlarged. 

highly  specialized  than  in  the  seed  plants.     Particularly  in  the 
stouter-stemmed  species,   the  fibro-vascular  system  sometimes 


THE  PrERIDOPffVTA 


331 


becomes  very  complex  and  develops  several  concentric  rings  of 
bundles,  the  members  of  which  are  connected  with  one  another 
in  an  intricate  fashion.  Masses  of  heavy-walled  sderenchyma 
cells  are  often  formed  in  pith  and  cortex  and  aid  in  maintaining 
the  rigidity  of  the  stem. 

The  sporangia  (Fig.  201)  are  borne  on  the  back  or  dorsal  surface 
of  the  leaf  in  definite  clusters  (Fig.  199)  known  as  fruit ing-dots 


Fig.   204. — Young  sporophyte  of  a  fern,  which  has  developed  from  a  fertilized 
egg,  growing  out  of  its  parent  gametophyte. 

or  sori  (singular,  sorus).  Each  sorus  is  usually  covered  until 
maturity  by  a  fold  of  thin,  skin-like  tissue,  the  indusium  (Fig. 
199),  which  arises  from  the  leaf  surface.  The  individual  sporan- 
gium is  very  small  in  comparison  with  those  of  bryophytes  and 
produces  only  a  few  spores.  In  most  cases  it  displays  around  its 
wall  a  characteristic  ring  of  cells,  the  annidus  (Fig.  201),  which  is 
so  constructed  that  upon  drying  it  contracts  like  a  spring,  finally 


332 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


rupturing  the  sporangium  wall  and  forcibly  ejecting  the  spores. 
The  shape  and  position  of  the  sorus  and  indusium,  as  well  as  the 
type  of  annulus,  vary  greatly  among  the  different  groups  of  ferns 
and  serve  as  useful  characters  by  which  to  distinguish  genera  and 
families. 

The  spores  germinate  into  a  thin,  small,  thallus-like  gameto- 
phyte  or  prothaUus  (Fig.  202)  which  possesses  chlorophyll  and  is 


Fig.  205. — Graphic  representation  of  the  life-history  of  a  fern.  1,  the  fern 
plant  or  sporophyte,  bearing  sori,  or  clusters  of  sporangia,  on  its  leaves.  2,  a 
sporangium.  3,  a  tetrad  of  young  spores.  4,  the  four  mature  spores  which  have 
come  from  the  tetrad  shown  in  3.  5,  a  spore  germinating  into  a  young  gameto- 
phyte.  6,  mature  gametophyte,  bearing  sexual  organs.  7,  archegonium.  7a, 
antheridium.  8,  egg  cell  or  female  gamete.  8a,  sperm,  or  male  gamete.  9, 
fertilized  egg.  10,  young  sporophyte  growing  out  of  a  fertilized  egg,  the  whole 
still  attached  to  the  remains  of  the  gametophyte. 


somewhat  heart-shaped  in  outline,  though  its  form  varies  con- 
siderably. It  is  rarely  more  than  a  few  millimeters  in  diameter 
and  lies  flat  upon  the  surface  of  the  soil,  to  which  it  is  attached 
by  delicate  rhizoids  growing  from  the  under  surface.  Plentiful 
moisture  and  a  partially  shaded  situation  are  necessary  for  the 
successful  growth  of  a  fern  prothallus.  The  sexual  organs  (Fig. 
203)  are  produced  on  the  under  surface,  the  archegonia  near  the 
"notch"  and  the  antheridia  farther  back,  among  the  rhizoids. 
The  archegonia  are  much  smaller  than  those  of  bryophytes  and 
only  their  necks  project  above  the  surrounding  tissue.     They 


THE  PTERIDOPJIYTA  333 

appear  when  tlu?  piothallu.s  is  fully  grown  and  considerably  after 
the  antheridia  have  liberated  their  sperms.  At  maturity,  the 
neck  of  the  archegonium  opens  and  the  neck-canal  cells  break 
down,  producing  a  substance  attractive  to  the  sperms.  The 
antheridium  is  also  very  much  smaller  and  simpler  than  it  is 
among  bryophytes.  Its  wall  consists  of  but  three  cells — a  cover- 
cell,  a  circular  cell  which  forms  the  main  wall,  and  a  funnel- 
shaped  basal  cell.  The  contents  of  the  antheridium  divides  into 
a  large  number  of  sperms,  each  possessing  a  tuft  of  cilia  by  which 
it  can  swim  about  in  a  thin  film  of  water  (Fig.  203).  A  sperm 
enters  an  archegonium  and  there  effects  fertilization,  after  which 
the  fertilized  egg,  by  repeated  cell  divisions,  forms  a  mass  of 
tissue  which  gradually  becomes  differentiated  into  the  body  of  the 
young  sporophyte.  This  soon  develops  a  vigorous  root  and 
shoot  (Fig.  204)  and  grows  into  the  mature  fern  plant.  The  life- 
cycle  of  a  fern  is  graphically  represented  in  Fig.  205. 

The  order  includes  nearly  3,000  species  and  its  members 
are  widely  distributed  over  the  globe,  being  particularly  rich  in 
species  and  individuals  throughout  all  tropical  regions.  It 
is  by  far  the  largest  group  of  pteridophytes  and  since  early 
times  has  occupied  an  important  place  in  the  earth's  vegetation. 

2.  Ophioglossales  or  Adder's  Tongues. — Here  are  placed  a  small 
group  of  fern-like  plants  which  are  of  interest  to  botanists  from 
their  possession  of  certain  characteristics  markedly  different 
from  those  of  other  ferns.  A  single  leaf,  simple  in  the  adder's 
tongue  fern  but  typically  fern-like  in  the  rest  of  the  order,  is 
produced  each  season  by  the  subterranean  stem.  From  the 
petiole  of  this  leaf  arises  a  spore-bearing  stalk  crowned  with  a 
cluster  of  heavy-walled  sporangia,  very  different  in  type  from  the 
thin-walled  structures  of  true  ferns.  The  gametophyte  is  thick 
and  tuberous,  and  is  partly  subterranean.  The  Ophioglossales 
are  often  placed  by  themselves  in  a  separate  class. 

3.  Hydropteridales  or  Water  Ferns. — This  is  another  small 
group,  chiefly  important  for  its  specialized  method  of  reproduc- 
tion. Its  sporophyte  is  aquatic  and  bears  little  or  no  resemblance 
to  that  of  the  true  ferns.  Marsilia  (Fig.  206),  the  clover-leaf 
fern,  is  the  commonest  representative.  From  the  petiole  of  its 
curious  four-lobed  leaf  arise  one  or  more  bean-like  sporocarps 
containing  sporangia  of  two  soYts;t\ie megasporangia,  each  produc- 
ing a  single  large  megaspore,  and  the  microsporangia,  each 
producing  a  group  of  smaller  microspores.     These  are  dispersed  in 


334 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


the  water  and  there  germinate.  The  megaspore  produces  a  small 
female  gametophyte  still  contained  largely  within  the  thick 
spore  wall,  and  at  the  point  where  the  wall  bursts  a  single  archego- 
nium  appears.  The  microspore  gives  rise  to  a  single  antheridium, 
which  liberates  into  the  water  a  group  of  sperms.  The  young 
sporophyte  which  develops  from  the  fertilized  egg  is  nourished 


Fig.  206.  Fig.  207. 

Fig.  206. — Marsilia.  The  spore-fruits  or  sporocarps  (s)  growing  out  from  the 
petiole  of  the  leaf  contain  microsporangia  and  megasporangia.  (From  Stras- 
burger,  after  Bischoff). 

Fig.  207. — A  club-moss  (Lycopodium) .  The  underground  stem  has  sent  up 
a  much-branched  shoot  on  which  are  the  small,  scale-like  leaves.  Upon  this 
are  borne  two  groups  of  cones. 

for  a  time  on  the  abundant  supply  of  food  stored  in  the  female 
gametophyte,  but  soon  becomes  independent  through  the 
establishment  of  a  root  and  leaf  of  its  own.  This  heterosporous 
type  of  reproduction  is  the  highest  found  among  the  Filicineae. 
Lycopodineae  or  Club  Mosses. — This  group  is  by  no  means  as 
rich  in  species  as  the  ferns.  The  sporophyte  (Fig.  207)  has  a  well 
developed  stem,  typically  prostrate  or  subterranean  but  sending 
up  numerous  erect  branches  which  sometimes  reach  a  decimeter 
or  more  in  height.  In  contrast  to  the  ferns,  the  leaves  are  very 
small,  numerous,  and  crowded  closely  on  the  stems,  presenting  a 


Tlll<:  I'TERIDOPHYTA  335 

moss-like  appearance  which  has  given  the  common  name  to  the 
group.  The  internal  structure  of  the  stem  in  Lijcopodium  is 
unique,  for  its  fibro-vascular  system  is  a  solid,  pithless  core,  made 
up  of  alternating  Imnds  of  wood  and  bast  extending  across  the 
central  cylindcM-  (Fig.  208).  The  anatomy  of  the  other  genera  is 
much  simpler. 

Sporangia  are  few  and  large  as  compared  with  those  of  the 
ferns,   and  are  borne  on  the  upper  or  ventral  leaf-surface.     In 


Fig.  208. — Transverse  section  (diagrammatic)  of  the  stem  of  Lrjcopodium. 
The  fibro-vascular  cylinder  consists  of  alternating  bands  of  wood  and  bast. 
From  this  cylinder  a  small  leaf-trace  departs  to  each  leaf.  Wood  black,  bast 
dotted. 

the  simpler  species,  a  sporangium  may  arise  on  an  ordinary  vege- 
tative leaf  but  in  most  cases  these  spore-bearing  leaves  (which 
here,  as  elsewhere  among  the  higher  plants  are  known  as  sporo- 
phylls)  become  stout  and  scale-like,  and  are  grouped  in  a  cone  or 
strohilus  at  the  tip  of  a  branch.  The  two  main  orders  Lycopodia- 
les  and  Selaginellales  are  distinguished  chiefly  by  their  methods 
of  reproduction. 

1.  Lycopodiales  (Fig.  209). — These  are  homosporous  plants, 
the  spores  which  they  produce  being  all  of  one  sort,  as  in  the 
Filicales.  The  gametophytes  vary  considerably  but  tend  to 
develop  a  stout  tuberous,  subterranean  portion,  which  may  be 
surmounted  ])y  a  green  aerial  region  on  which  the  sexual  organs 
luv  borne  (Fig.  210).     These  are  larger  and  bettor  developed  than 


33G 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


among  the  ferns,  and  the  sperms  resemble  those  of  bryophytes  in 
being  bicihate.  After  fertiUzation  the  young  sporophytc  is 
carried  rather  deeply  into  the  prothallial  tissue  by  a  long  cell,  the 
suspensor,  and  develops  through  its  early  stages  largely  at  the 
expense  of  the  gametophyte.  To  this  order  belongs  the  large 
genus  Lycopodiutn,  the  familiar  club  moss  or  ground  pine. 


Fig.  209. — Lycopodium.  A,  part  of  a  plant  of  Lycopodium  annotinum  showing 
prostrate  stem  and  leafy,  erect  shoots  bearing  cones  or  strobili  (s).  B,  sporophyll 
or  cone-scale,  bearing  a  sporangium  on  its  upper  surface.  C,  one  of  the  very 
numerous  spores  produced  in  this  sporangium,  greatly  enlarged. 


2.  Selaginellales  (Fig.  211). — This  order  is  represented  by  the 
genus  Selaginella,  which  resembles  Lycopodium  rather  closely  in 
vegetative  structures  but  differs  from  that  genus  in  being  hetero- 
sporous.  Certain  of  the  sporangia  (the  megasporangia.  Fig. 
211,  B)  produce  four  large  megaspores  each,  and  the  others  (the 
microsporangia.  Fig.  211,  C)  produce  an  abundance  of  much 
smaller  microspores.  The  history  of  the  gametophytes  is  in 
many  ways  like  that  described  for  the  water  ferns.  The  megas- 
pore  produces  a  small  mass  of  cells,  most  of  which  are  still  retained 


THE  PTEJilDOPIIYTA 


337 


an 


•      .^^ 

i 

Fig.  210. — Gametophyte  of  Lycopodium,  showing  the  stout,  subterranean, 
tuber-like  portion  and  the  upper  region  in  which  antheridia  (an)  and  archegonia 
{ar)  are  produced.      (From  Strasburger,  after  Bruchmann). 


Fig.  211. — Selagmella.  .4,. leafy  branch  bearing  strobili  (s).  B,  megasporo- 
phyll  or  cone-scale  bearing  a  megasporangium.  One  of  the  four  megaspores 
produced  in  this  sporangium  is  shown  at  the  left.  C,  microsporophyll  or  cone- 
scale  bearing  a  microsporangium.  One  of  the  many  microspores  produced  in  this 
sporangium  is  shown  at  the  left. 
22 


338 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


within  the  remains  of  the  stout  megaspore  wall  (Fig.  212). 
On  the  exposed  tissue,  a  group  of  archegonia  appear.  Each 
microspore  forms  a  single  antheridium  in  which  a  group  of 
biciliate  sperms  develops.  The  young  sporophyte  is  thrust 
deeply  into  the  tissues  of  the  gametophyte  until  it  has  begun  its 
differentiation.  Such  a  life  history  as  this  is  clearly  a  step 
in  the  direction  of  seed  production. 

The  genus  Isoetes,  the  Quill  wort  (Fig.  213),  is  usually  included 
among  the  lycopods  although  its  remarkable  characteristics  have 


Fig.  212. — Female  gametophyte  of  Selaginella.  The  stout  wall  of  the  micro- 
spore still  encloses  part  of  the  gametophyte.  At  the  right  is  an  archegonium;  at 
the  center  and  left,  young  embryos  which  have  arisen  from  fertilized  eggs  in 
other  archegonia.  The  embryo  is  carried  down  into  the  tissue  of  the  gameto- 
phyte by  an  elongated  cell,  the  suspensor.  In  the  larger  embryo,  the  shoot  (at 
right)  and  root  (at  left)  are  beginning  to  become  differentiated,  as  well  as  the 
large,  absorbing  foot,  at  the  lower  left.      (Mainly  after  Pfeffer). 


caused  some  botanists  to  place  it  in  a  distinct  order.  The  plants 
grow  in  water  or  very  moist  places  and  each  consists  of  a  tuft  of 
long,  quill-like  leaves,  arising  from  a  short  and  flattened  stem. 
In  the  hollow  bases  of  these  leaves  the  sporangia  are  borne. 
Isoetes  is  heterosporous,  its  gametophytes  being  similar  in  general 
structure  to  those  of  Selaginella  except  that  the  sperms  have 
many  cilia. 

The  lycopods  were  particularly  prominent  in  the  forests  of  the 
Coal  Period,  the  great  tree-like  lepidodendrids  and  sigillarians 
belonging  to  this  order.  It  is  noteworthy  that  these  ancient 
members  of  the  group  possessed  a  cambium  and  well  developed 
secondary  wood,  tissues  which  are  quite  absent  in  living  lycopods. 


THE  PTERIDOI'IIYTA 


339 


Equisetineae  or  Horsetails. — This  vory  distinct  class  consists  of 
but  one  order,  tlie  Equisetales,  and  this  of  but  the  single  genus 
Equisetum,  the  horsetails  or  scouring  rushes  (Fig.  214).  Some  of 
its  species  grow  in  dry  sterile  soil  and  others  in  marshy  situa- 
tions. From  a  perennial  rootstock  arise  the  very  characteristic 
stems,  which  are  jointed,  ridged,  and  hollow.     Leaves  are  repre- 


Fio.  213. — Isoetes.     A,    general    appearance   of    the    plant.     B,  a  megaspore. 
C,  a  microspore. 


sented  merely  by  a  circle  of  scales  which  surround  the  stem  at 
each  joint  or  node,  the  green  stems  carrying  on  most  of  the  pro- 
cess of  photosynthesis.  Aside  from  the  large  central  air  space, 
a  smaller  one  occurs  just  inside  each  furrow  in  the  stem  (Fig.  215). 
Opposite  each  ridge  is  a  small  and  very  poorly  developed  fibro- 
vascular  bundle,  the  stoutness  and  strength  of  the  stem  being  due 
chiefly  to  the  layers  of  heavy-walled   sclcrenchyma  which  it 


340 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fig.  214 — Equisctum  anemc  A  pHnt  producing  from  its  undergiound 
rootstock  several  fertile  stems  (A),  teiminating  in  cones  (1),  and  a  sterile,  much- 
branched  shoot  B.  C,  one  of  the  sporophylls  from  the  cone,  bearing  a  group  of 
sporangia.  D,  the  same,  with  the  sporangia  ruptured  and  their  spores  shed. 
E,  F,  and  G,  spores,  greatly  enlarged,  with  the  elaters  in  various  positions. 
(From  Strasburger,  after  Wossidlo). 


THE  PTERIDOPIIVrA 


341 


contains.     Across  the  stem  at  each  node  extends  a  sohd  parti- 
tion or  diaphragm.     The  stems  may  be  branched  or  unbranched. 
As  in  the  lycopods,  the  sporangia  are  borne  in  terminal  cones 
(Fig.  214).     The  sporophylls,  however,  are  not  at  all  leaf -like  but 


Fig.  215. — Transverse  section  (diagrammatic)  of  the  stem  of  Equisetum. 
Note  the  large  central  cavity,  the  air  chambers  opposite  the  stem  furrows,  and 
the  smaller  ones  in  the  bundles.  The  fibro-vascular  system  is  much  reduced, 
consisting  of  a  ring  of  small  bundles,  each  with  two  minute  arms  of  wood  and  a 
patch  of  bast  between.     Wood  black,  bast  dotted. 


Fig.  216. — Gametophytes  of  Equisetum.  A,  male  gametophyto,  showing 
antheridia.  B,  sperms.  C,  female  gametophyte,  showing  the  long,  branching 
lobes  with  archegonia  at  their  bases.     (A  and  C  after  Ilofmcister,  B  after  Schachl) . 

are  somewhat  shield-shaped  and  project  outward  at  right  angles 
to  the  cone  axis.  On  their  under  or  inner  surfaces  are  borne 
a  row  of  sporangia.     The  spores  are  all  alike  externally,  but  a 


342  BOTANY:  PRINCIPLES  AND  PROBLEMS 

given  spore  will  generally  produce  either  a  strictly  male  or  a 
strictly  female  gametophyte.  Attached  to  each  spore  are  four 
thread-hke  structures,  the  elaters  (Fig.  214),  which  coil  tightly 
around  it  when  moist  but  expand  when  dry,  and  thus  aid  in  spore 
dispersal. 

The  gametophyte  (Fig.  216)  is  an  irregular  thallus  which 
develops  chlorophyll,  shows  no  tendency  toward  the  subterranean 
habit  of  the  lycopod  prothallus,  and  is  provided  with  long, 
branching  lobes.  The  two  sexes  are  usually  on  separate  plants 
and  the  gametophytes  are  therefore  said  to  be  dioecious.  The 
sexual  organs  resemble  in  general  those  of  other  pteridophytes. 
The  antheridia  occur  at  the  tips  of  the  lobes  and  the  archegonia 
at  their  bases.  Each  sperm  has  a  tuft  of  cilia.  In  embryonic 
development  no  suspensor  is  formed,  and  the  whole  process 
considerably  resembles  that  found  among  the  ferns. 

The  Equisetineae,  like  the  Lycopodineae,  were  represented 
in  earlier  periods  of  the  earth's  history  by  large,  tree-like  species 
with  well  developed  cambium  and  secondary  wood.  Many  of 
these  were  also  heterosporous. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

769.  Which  of  the  three  groups  of  pteridophytes  is  the  most  ancient, 
in  your  opinion?     Why? 

770.  What  other  groups  of  plants  already  described  (aside  from  the 
pteridophytes)  were  once  probably  very  abundant  but  are  now  repre- 
sented by  only  a  comparatively  small  number  of  species? 

771.  Why  is  asexual  reproduction  so  common  among  the  pterido- 
phytes? 

772.  Do  you  think  that  heterospory  has  arisen  more  than  once  in  the 
evolution  of  the  plant  kingdom?     Explain. 

773.  Which  do  you  think  is  more  primitive  among  pteridophytes, 
thin- walled  sporangia  or  thick- walled  ones?     Why? 

774.  Can  you  suggest  a  reason  for  the  fact  that  ferns  are  now  so 
much  more  abundant  than  club  mosses  or  horsetails? 

775.  Why  are  there  no  tree-ferns  in  temperate  climates? 

776.  How  does  the  trunk  of  a  tree-fern  differ  from  that  of  an  ordinary 
tree? 

777.  Why  does  not  the  trunk  of  a  tree-fern  make  good  lumber? 


THE  I'TKIUDOl'IIYTA  343 

778.  Arc  coinniou  f(M-ns  annuals  or  perennials?     Why? 

779.  Of  wliat  advantage  to  the  fern  plant  is  the  indusium? 

780.  Of  what  advantage  is  it  to  the  fern  plant  to  have  the  archegonia 
and  the  antheridia  on  the  same  gametophyte  mature  at  different  times? 

781.  In  what  particulars  do  the  Ophioglossales  resemble  the  ferns? 
In  what  do  they  resemble  the  club  mosses? 

782.  Do  you  think  that  a  cone  is  more  pi'iinitiv(>  tlian  a  group  of 
leaf-like  sporophylls  or  not?     Why? 

783.  Of  what  advantage  to  the  plant  is  it  to  have  its  sporophylls 
grouped  into  a  cone  rather  than  scattered  along  the  stem? 

784.  How  do  a  typical  fern  and  a  typical  horsetail  differ  in  the  environ- 
ment to  which  they  are  best  adapted? 

785.  What  are  the  advantages  of  a  subterranean,  saprophytic  gameto- 
phyte over  the  free-living  one  characteristic  of  most  ferns?  Which 
form  do  you  think  is  the  more  primitive?     Why? 

786.  In  what  particulars  does  Isoetes  resemble  the  ferns?  In  what 
does  it  resemble  the  club  mosses? 

787.  In  what  particulars  do  the  Equisetales  resemlile  the  Filicales? 
In  what  do  they  resemble  the  Lycopodiales? 

788.  Of  what  significance  is  the  fact  that  the  gametophytesof  ^'gwise- 
tnm  are  usually  dioecious? 

789.  How  do  the  elaters  in  the  sporangium  of  Equisetum  aid  in  spore 
dispersal? 

REFERENCE  PROBLEMS 

133.  Of  what  cconoinic  importance  are  the  pteridophytes? 

134.  Construct  for  Lijcopodium  a  graphic  life-cycle  similar  to  that  given 
in  the  text  for  a  fern  (Fig.  205). 

135.  Construct  a  similar  grapliic;  life-cycle  for  Sddgiiiclld. 

136.  Construct  a  similar  graphic  life-cycle  for  Equisetum. 

137.  Give  the  derivations  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Heterosporous  Indusium  Prothallus 

Sorus  Annulus  Sporophyll 

Strobilus 


CHAPTER  XVII 
THE  SPERMATOPHYTA 

This  enormous  group,  to  which  the  early  portions  of  our  text 
were  largely  devoted,  includes  the  most  famihar  and  abundant 
part  of  the  earth's  plant  population.  Directly  or  indirectly, 
the  seed  plants  furnish  almost  all  the  food  supply  of  the  human 
race,  all  of  its  timber  and  fiber  plants,  and  a  great  majority  of 
the  animal  and  vegetable  products  which  form  the  basis  of  our 
civihzation.  It  is  this  division  of  the  plant  kingdom  which  is 
present  most  intimately  in  the  thoughts  and  lives  of  men,  and 
which  for  a  long  time  provided  practically  the  entire  subject- 
matter  for  the  science  of  botany. 

In  vegetative  characters  the  spermatophytes  are  not  remarkably 
different  from  the  pteridophytes.  They  display  the  same 
vigorous  development  of  root,  stem,  and  leaf,  and  although  they 
commonly  show  a  greater  specialization  and  differentiation  of 
their  tissues,  particularly  in  the  higher  groups,  the  fundamental 
plan  established  in  the  ferns,  with  its  emphasis  on  a  well  devel- 
oped fibro-vascular  system,  is  retained  and  further  developed. 
Growth  of  the  stem  in  diameter  by  means  of  an  active  cambium 
occurs  in  most  seed  plants,  although  it  is  much  reduced  or 
absent  in  the  more  delicate  herbaceous  species. 

The  Origin  of  the  Seed. — The  distinguishing  feature  of  the 
spermatophytes,  as  their  name  indicates,  is  the  development  among 
them  of  a  new  reproductive  structure,  the  seed.  In  an  earlier 
chapter  we  have  outlined  briefly  the  evolution  of  the  seed-habit 
and  its  significance;  but  in  view  of  the  greater  familiarity  with 
the  lower  plants  which  we  now  possess,  it  will  perhaps  be  worth 
while  to  describe  the  seed  and  its  origin  somewhat  more  fully 
before  we  take  up  in  detail  the  various  plant  groups  in  which 
this  structure  occurs. 

Relation  to  Structures  in  the  Lower  Groups. — The  seed-producing 
habit  is  a  direct  development  from  such  a  condition  of  heterospory 
as  has  been  attained  by  the  higher  pteridophytes.  It  will  be 
remembered  that  in  the  water  ferns,  in  Selaginella,  and  in  Isoetes, 

344 


THE  SPERM  A  TOP  11 Y  TA 


345 


Fig.  217. — Development  of  the  female  gametophyte  and  .seed  in  a  gyninospcrm 
(Pine),  Longitudinal  sections  through  the  base  of  the  cone-scale.  A,  young 
ovule,  consisting  of  an  integument  (1)  and  a  megasporangium  or  nucellus  (2), 
within  which  is  a  row  of  four  megaspores  which  have  developed  from  a  megaspore 
mother-cell.  B,  three  of  the  megaspores  have  aborted,  the  fourth  is  enlarging. 
C,  the  megaspore  has  germinated  into  a  young  gametophyte  or  em))ryo-sac  (3), 
which  now  consists  of  a  layer  of  free  nuclei  surrounding  a  large  vacuole.  D,  the 
young  sac  has  developed  into  the  mature  female  gametopliyte  (3),  the  bulk  of 
which  consists  of  endosperm.  At  one  end  are  two  archegonia  (4)  within  each  of 
which  is  an  egg-cell.  Pollen  grains  have  entered  the  micropyle  and  alighted  on 
the  tip  of  the  nucellus.  E,  fertilization.  Two  pollen  grains  have  germinated, 
sent  their  tubes  down  through  the  nucellus,  and  discharged  their  contents  into  the 
two  archegonia,  in  each  of  which  one  of  the  male  nuclei  is  fusing  with  the  egg 
nucleus  (5).  F,  from  the  fertilized  egg  have  grown  two  young'embryos  (6),  one 
larger  than  the  other,  which  have  been  pushed  down  into  the  middle  of  the  endo- 
sperm. G,  mature  seed.  The  integument  of  the  ovule  has  developed  into  the 
seed  coat,  the  micropyle  has  closed,  the  endosperm  has  become  greatly  enlarged, 
the  nucellus  has  almost  disappeared  and  the  embryo  has  grown  to  its  full  size. 
The  smaller  embryo  in  F  failed  to  develop.  This  seed  will  now  detach  it.self 
from  the  cone-scale  and  under  favorable  conditions  will  produce  a  new  plant. 


346  BOTANY:  PRINCIPLES  AND  PROBLEMS 

the  spores  are  not  uniform  but  that  microspores  and  megaspores, 
borne  in  separate  sporangia  and  on  separate  sporophylls,  germi- 
nate into  male  and  female  gametophytes,  respectively.  The 
seed  plants  are  likewise  heterosporous.  As  in  the  lower  groups, 
they  develop  microspores  (now  called  pollen  grains).  These 
are  borne  in  a  microsporangium  (now  called  an  anther),  arising 
from  a  microsporophyll  (now  called  a  stamen).  No  very  radical 
change  is  evident  here,  but  in  the  case  of  the  female  structures 
we  find  some  marked  innovations. 

The  Ovule  and  Its  Contents  (Figs.  217  and  229).— The  mega- 
sporangium,  now  known  as  the  nucellus,  produces  only  one 
functioning  megaspore,  for  the  other  three  members  of  the  tetrad 
which  begin  to  develop  soon  disappear.  Furthermore,  the 
sporangium  does  not  burst  and  liberate  this  spore  but  retains  it, 
instead,  and  allows  it  to  germinate  and  produce  the  female 
gametophyte  within  the  tissues  of  the  sporangium  (nucellus), 
nourished  by  the  parent  sporophyte.  The  female  gametophyte 
(now  called  the  embryo-sac)  is  a  small,  roundish  group  of  cells 
filled  with"  food  and  bearing  at  one  end  one  or  more  archegonia 
or  structures  comparable  to  them.  The  whole  is  embedded  in 
the  tissue  of  the  nucellus  and  is  never  freely  exposed.  Among 
the  highest  forms  it  suffers  such  reduction  that  resemblance  to  a 
gametophyte  becomes  very  faint.  The  nucellus  and  its  enclosed 
embryo-sac  are  completely  surrounded  and  protected  by  one 
or  two  coats  or  integuments  except  for  a  small  opening,  the 
micropyle,  which  occurs  just  opposite  the  point  where  the  arche- 
gonia are  borne  in  the  embryo-sac  beneath.  The  whole  struc- 
ture— integument,  nucellus,  and  embryo-sac — is  known  as  the 
ovule,  and  after  fertilization,  for  which  it  is  now  prepared,  the 
ovule  will  develop  into  a  seed.  It  is  closely  attached  to 
the  megasporophyll,  which  is  now  called  a  carpel. 

Pollen  (Fig.  218). — The  microspore  or  pollen  grain,  produced 
in  the  microsporangium,  in  the  mean  time  germinates  and 
produces  within  itself  the  male  gametophyte.  This  is  greatly 
reduced  and  consists  at  most  of  a  very  few  cells,  in  the  higher 
forms  of  only  two — the  generative  cell  and  the  tube-nucleus. 
These  are  all  that  remains  of  the  male  sexual  generation.  Germi- 
nation of  the  microspore  usually  takes  place,  at  least  in  part, 
before  it  is  liberated  by  the  breaking  open  of  the  anther  wall. 
After  this  event,  the  pollen  grain  is  transferred,  by  wind,  insects 
or  other  means,  either  directly  to  the  ovule  or  to  a  receptive 


THE  SPERM  A  TOPII  VTA 


347 


structure  (the  stigma)  which  is  nearby.  Since  the  egg  cells 
are  buried  in  the  nuccllar  tissue,  it  is  evident  that  the  male 
gametes  cannot  approach  them  directly,  as  in  the  lower  plants, 
and  a  new  structure  has  accordingly  been  developed  which 
conveys  them  to  the  egg.  This  is  the  pollen-tube.  It  arises 
from  the  pollen-grain  as  a  slender,  thin-walled  projection  and 
into  it  the  contents  of  the  gi'ain  passes,  led  by  the  tube-nucleus. 


Fig.  218. — Development  of  the  male  gametophyte  in  a  gymnosperm  (Pine). 
A,  longitudinal  section  (diagrammatic)  of  a  staminate  or  "male"  cone.  B,  one 
of  the  sporophylls  from  A,  much  enlarged.  The  sporangium  is  filled  with  micro- 
spores or  pollen,  in  various  stages  of  development.  C,  section  across  the  sporo- 
phyll  at  right  angles  to  that  in  B,  showing  the  two  sporangia  which  are  borne  on 
each  sporophyll.  D,  a  mature  pollen  grain.  The  outer  wall  (in  the  case  of  pine 
pollen)  is  p^iffed  out  somewhat  at  two  points,  forming  balloon-like  "wings" 
which  add  to  the  buoyancy  of  the  grain.  The  single  nucleus  of  the  megaspore 
has  now  divided  to  form  the  generative  cell,  lying  next  the  wall,  and  the  tube 
nucleus,  in  the  center  of  the  cell.  E,  a  germinating  pollen  grain.  The  tube 
nucleus  follows  close  behind  the  end  of  the  growing  tube.  The  generative  cell 
has  divided  into  a  stalk  cell  (lighter)  and  a  body  cell  (darker).  F,  the  end  of  the 
pollen-tube  just  before  fertilization.  The  body  cell  has  developed  into  the 
two  male  cells  or  gametes,  which  have  now  come  down  the  tube  and  are  ready  to 
effect  fertilization. 


Fertilization  and  Seed-production  (Figs.  217  and  229). — The 
poll(Mi  tube  grows  rapidly  and  penetrates  the  tissues,  much  as  a 
fungus  filament  penetrates  the  tissues  of  its  host,  until  it  reaches 
the  embryo-sac  and  the  egg-cells.  A  large  nucleus  in  the  tube, 
often  the  only  one  at  this  point  except  the  tube  nucleus,  now 
divides  into  two  male  gametes,  and  as  the  end  of  the  tube  bursts 


348  BOTANY:  PRINCIPLES  AND  PROBLEMS 

into  the  sac  one  of  these  unites  with  an  egg.  The  fertihzed 
egg  now  divides  and  grows  into  a  young  sporophyte,  the  embryo, 
possessing  a  primitive  root,  stem,  and  leaves.  The  embryo  soon 
stops  its  growth  and  becomes  dormant,  embedded  in  the  tissues 
of  the  sac  which  are  by  now  filled  with  reserve  food  or  endosperm. 
The  integument  in  the  mean  time  has  developed  into  the  tough 
seed  coat  and  the  whole  structure  soon  separates  from  the  mother 
plant  as  a  mature  seed.  Under  favorable  conditions  of  tempera- 
ture and  moisture  this  seed  will  germinate,  the  embryo  breaking 
out  through  the  seed  coats  and  estabhshing  itself  in  the  soil 
as  a  new  plant  which  grows  for  a  time  at  the  expense  of  the  stored 
food  but  soon  becomes  independent. 

The  Advances  from  Pteridophytes  to  Seed  Plants. — The  essen- 
tial advances  made  by  seed  plants  over  the  higher  pteridophytes 
are  therefore:  (1)  The  retention  of  the  megaspore  within  the 
megasporangium  and  its  germination  there  into  the  female 
gametophyte;  (2)  the  enclosure  of  the  sporangium  and  sac  by  a 
new  structure,  the  integument;  (3)  the  transference  of  the 
reduced  male  gametophyte  directly  to  the  vicinity  of  the  female 
gametophyte,  to  which  the  male  gametes  obtain  access  by  another 
new  structure,  the  pollen-tube;  (4)  the  development  of  the  young 
sporophyte  in  contact  with,  and  at  the  expense  of,  the  parent 
sporophyte,  and  (5)  its  final  release,  dormant,  well  supplied  with 
food,  and  protected  by  a  heavy  coat.  It  is  noteworthy  that  the 
reversal  of  the  reproductive  situation  as  we  find  it  in  the  bryo- 
phytes  is  now  complete,  for  instead  of  the  sporophyte  being 
attached  to  the  gametophyte,  the  gametophyte  (and  even  the 
succeeding  sporophyte)  is  here  attached  to  the  parent  sporo- 
phyte. Indeed,  in  the  most  advanced  types  both  gametophytes 
are  so  much  reduced  that  little  beside  the  sexual  cells  remains, 
and  the  alternation  of  generations  has  practically  disappeared, 

The  Flower. — The  sporophylls  of  seed  plants  tend  to  be 
arranged  in  distinct  clusters  on  short  branches.  In  the  lower 
members  of  the  group  these  clusters  are  entirely  similar  to  the 
cones  of  some  of  the  pteridophytes,  but  higher  up  a  very  special- 
ized shoot,  commonly  called  the  flower,  has  been  evolved.  In 
its  fully  developed  form  this  contains  not  only  the  stamens  and 
carpels,  but  modified  leaf -like  structures  for  protection  of,  the 
sexual  organs  and  for  attraction  of  insects.  From  the  possession 
of  this  structure  the  spermatophytes  are  sometimes  called  the 
"flowering  plants." 


THE  SPERM ATOI'IIYTA  349 

To  describe  adequately  the  various  orders  of  which  this  huge 
division  is  composed  is  quite  impossible  within  the  limits  of  our 
text.  Aside  from  its  numerous  and  varied  living  members  it  also 
includes  a  great  number  of  fossil  types,  many  of  which  are  impor- 
tant in  reconstructing  for  us  the  steps  in  the  evolutionary  history 
of  the  seed  plants.  We  shall  attempt  to  present  the  salient 
characters  of  the  more  important  groups  only,  and  to  indicate 
their  probable  relationship  to  each  other  and  their  place  in  the 
phylogeny  of  the  whole  series. 

Gymnospermae  or  Gymnosperms. — Two  classes  of  seed  plants 
are  commonly  recognized,  the  gymnosperms  and  the  angiosperms, 
differing  chiefly  as  to  the  manner  in  which  the  ovules  are  borne. 
The  gymnosperms  are  the  most  ancient  of  seed  plants,  and  a 
varied  and  heterogeneous  group.  All  agree  in  possessing  ovules 
and  seeds  which  are  borne  openly  on  the  megasporophyll  or 
carpel,  freely  exposed  to  the  air  and  to  the  direct  contact  of 
pollen  grains,  and  not  inclosed  in  an  ovary  as  they  are  among  the 
angiosperms.  There  is  good  reason  to  believe  that  the  earliest 
gymnosperms  arose  from  fern-like  ancestors,  for  several  remark- 
able fossil  plants  from  the  Coal  Period  have  been  discovered 
which  possessed  typically  fern-like  foliage,  and  were  long  thought 
to  be  true  ferns,  but  which  are  now  known  to  have  borne 
undoubted,  though  primitive,  seeds.  This  group,  sometimes 
called  the  Cycadofilicales  or  cycad-ferns,  has  long  been  extinct, 
nor  have  any  of  its  near  relatives  survived.  The  most  primitive 
living  gymnosperms  are  the  Cycads. 

1.  Cycadales  or  Cycads. — Cycad  stems  are  typically  stout  and 
unbranched,  and  bear  at  the  top  a  crown  of  large,  pinnate  leaves 
(Fig.  219).  A  few  species  have  tall,  columnar  trunks  and  thus 
superficially  resemble  tree  ferns  or  palms.  Internally,  the  stem 
possesses  a  large  pith  and  cortex  but  its  woody  tissue  is  rather 
weakly  developed,  although  the  fibro-vascular  system  is  often 
complicated  by  the  occurrence  of  several  concentric  rings  of 
bundles  instead  of  a  single  ring. 

The  male  and  female  sexual  structures  occur  on  separate  plants, 
and  the  sporophylls  are  borne  in  terminal  cones  (Fig.  220).  In 
the  genus  Cycas,  the  ovulate  sporophylls  ("female"  cone  scales) 
are  large  and  lobed,  showing  a  slight  resemblance  to  foliage 
leaves,  and  bear  ovules  along  their  edges  (Fig.  221).  In  the  other 
members  of  the  order  the  cones  are  more  compact  and  the  sporo- 
phylls or  cone  scales  have  lost  their  leaf-like  character.     Each 


350  BOTANY:  PRINCIPLES  AND  PROBLEMS 

microsporophyll  (Fig.  222)  bears  many  sporangia  or  anthers  and 
produces  a  large  amount  of  pollen.  The  ovules  are  generally 
large  and  thick-walled,  and  the  megaspore,  arising  as  one  of  four 
potentially  spore-producing  cells  in  the  middle  of  the  nucellus, 
develops  into  a  large  embryo-sac.  At  the  tip  of  the  nucellus. 
just  under  the  micropyle,  a  large,  liquid-filled  pollen  chamber 
arises.     The  pollen  grain  enters  this  chamber  through  the  micro- 


FiG.  219. — Cycas  revohita,  one  of  the  Cycadales.     Male  plant  with  cone.     (Photo 
hyG.  S.  Torrey). 


pyle  and  there  germinates.  Its  two  male  cells  are  each  provided 
with  a  spiral  band  of  cilia  and  swim  about  in  the  liquid,  a  remark- 
able persistence  of  the  habit  of  swimming  sperms  which  harks 
back  through  pteridophytes  and  bryophytes  to  their  remote 
algal  ancestry.  A  pollen-tube  is  formed  and  penetrates  the 
adjacent  nucellar  tissue,  but  it  seems  rather  to  absorb  food  than 
to  assist  in  the  transference  of  the  male  cells,  for  the  pollen- 
chamber  gradually  enlarges  itself  until  it  reaches  the  embryo- 
sac,  where  one  of  the  sperms  enters  an  archegonium  and  effects 
fertilization. 

Cycads  are  confined  to  the  warmer  regions  of  the  globe.  They 
are  an  inconspicuous  group  today  but  in  earlier  ages,  notably  in 
the  Mesozoic  Era,  were  abundant  and  diversified.  Their 
close  relatives,  the  Bennettitales  (now  extinct),  were  for  a  time 


THE  SPERMArOPHYTA 


351 


among    the    most    conspicuous    of    plants    and    pi-oduced    some 
complex  flower-like  bisexual  reproductive  organs. 

Closely  related  to  the  cycads  is  the  Japanese  maidon-haii- 
tree,  Ginkgo,   usually  placed   in  a  separate  order  by  itself.     It 


Fig.   220. — Staminate  cone  of  Cycas  revoluta.      (Photo  by  G.  S.  Torny). 


resembles  the  cycads  in  the  possession  of  a  pollen-chamber  and 
of  motile  sperms,  but  differs  in  its  tree-like  habit  of  growth  and  in 
the  absence  of  typical  cones. 

2.  Coniferales  or  Conifers. — These  are  familiar  to  us  from  tlunr 
wide  distribution  in  temperate  zones  and  from  the  fact  that  they 
include  many  of  our  most  important  forest  trees.  The  vegetative 
body  diff(M-s  radically  from  that  of  the  cycads  for  it  is  usually 
a  tree,  with  a  straight  trunk  and  spreading  lateral  branches 
which  give  a  spire-like  shape  to  the  whole  (Fig.  8).  The  leaves 
are  typically  small,  evergreen  scales  or  needles.     The  bulk  of  t  he 


352 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


stout  woody  stem  is  secondary  wood,  laid  down  by  an  active 
cambium.  Its  water-conducting  cells  are  all  tracheids,  those 
produced  in  the  spring  being  comparatively  wide  and  thin-walled 


Fig.  221.  Fig.  222. 

Fig.  221. — Cycas.  Sporophyll  from  ovulate  cone,  showing  several  ovules 
attached  to  its  side.  One  of  these  has  developed  into  a  seed.  {From  Strasburger, 
after  Sachs). 

Fig.  222. — Cycas.  Sporophyll  from  staminate  cone,  showing  numerous 
pollen  sacs  or  microsporangia.     (From  Strasburger,  after  Richard) . 


* 

k 

Fig.  223. — Staminate  or  "male"  cones  of  the  pine. 

and  those  in  the  summer  nuich  narrower  and  thicker-walled 
(Fig.  63).  The  general  structure  of  the  vascular  tissues 
approaches  rather  closely  to  that  of  the  angiosperms, 


THE  SPERM  A  TOI'IIYTA 


353 


As  its  name  implies,  the  reproductive  structures  in  this  order 
are  typically  produced  in  cones.  The  microsporangial  (staminate 
or  "male")  cones  (Fig.  223)  are  short-lived  and  somewhat 
delicate  structures,  and  each  cone-scale  (stamen  or  microsporo- 
phyll)  bears  two  (rarely  more)  microsporangia  on  its  lower  or 
dorsal  surface,  in  which  the  microspores  or  pollen  grains  are 


Fig.  224. — Ovulate  or  "female"  cones  of  the  pine. 


developed  (Fig.  218).  The  pollen  is  in  all  cases  transferred  to  the 
ovules  by  wind.  Except  for  the  rather  small  group  Taxaceae, 
in  which  cone-scales  or  integuments  become  fleshy  at  maturity, 
the  megasporangial  (ovulate  or  "female")  cones  (Figs.  224  and 
225)  usually  become  hard  and  woody.  Each  cone-scale  bears  one 
or  two  ovules.  In  most  cases  the  embryo-sac  is  distinctly  smaller 
than  that  of  the  cycads  and  contains  fewer  cells  (Fig.  217).  The 
pollen  alights  on  the  nucellus  and  there  germinates  (Fig.  218). 
The  generative  cell  at  this  time  divides  into  two,  a  stalk  cell  and 
a  body  cell,  which  are  believed  to  represent  the  remains  of  an 
antheridium.  The  body-cell  in  time  follows  the  tube-nucleus 
down  the  pollen  tube  and  divides  into  two  male  cells,  one  of  which 
effects  fertilization  (Fig.  226).  No  pollen-chamber  is  formed,  but 
the  pollen-tube  conveys  the  male  cells,  which  are  non-motile. 


354 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


directly  to  the  archegonia.  After  fertilization  there  are  a  few 
divisions  of  the  egg  within  the  archegonium  itself,  and  the  young 
proembryo  thus  formed  is  then  carried  deeply  into  the  tissues  of 
the  embryo-sac  by  certain  of  its  upper  cells,  which  rapidly 
elongate.  In  this  position  it  develops  into  the  mature  embryo 
of  the  seed. 


Fig.  225.  Fig.  226. 

Fig.  225. — Longitudinal  section  (diagrammatic)  of  the  ovulate  or  "female" 
cone  of  pine.  Attached  to  the  base  of  each  scale  is  seen  an  ovule,  its  micropyle 
pointing  inward. 

Fig.  226. — Fertilization  in  a  conifer.  Archegonium  into  which  a  pollen-tube 
has  just  entered.  One  of  the  male  nuclei  may  be  seen  uniting  with  the  egg 
nucleus.     The  other,  left  behind  in  the  cytoplasm,  will  die.      (From  Sinnoll). 


Like  cycads,  the  conifers  are  an  ancient  group  and  are  promi- 
nent in  fossil  floras  since  Mesozoic  times.  Although  they  include 
only  about  350  species  today,  they  cannot  well  be  called  degenerate 
for  in  many  parts  of  the  forested  regions  of  both  the  north  and  the 
south  temperate  zones  they  contribute  to  the  vegetation  such 
notable  trees  as  the  pines,  spruces,  firs,  larches,  hemlocks,  cedars, 
cypresses,  and  many  others. 

3.  Gnetales. — Brief  mention  should  be  made  of  the  most  highly 
specialized  order  of  gymnosperms,  the  Gnetales,  which  consist  of 
three  genera  only;  a  tropical  climber,  a  desert  shrub,  and  an 


THE  SPERMATOPHYTA  355 

anomalous  desert  plant.  These  are  distinguished  from  other 
members  of  the  class  chiefly  by  the  possession  of  vessels  or  ducts 
in  the  wood  and  by  a  marked  reduction  in  the  female  gametophyte 
somewhat  similar  to  that  which  occurs  among  angiosperms.  It 
has  been  suggested  that  through  forms  related  to  the  Gnetales,  the 
angiosperms  may  perhaps  have  arisen  from  the  gymnosperms. 

Angiospermae  or  Angiosperms. — Angiosperms  differ  from 
gymnosperms  chiefly  in  the  fact  that  their  seeds  are  not  directly 
exposed  to  the  air  on  the  open  surface  of  a  scale  but  are 
enclosed  in  a  definite  case  or  vessel,  the  ovary. 

With  their  135,000  species,  their  highly  perfected  and  typically 
insect-pollinated  flowers,  their  enormously  diversified  plant  bodies, 
their  successful  invasion  of  all  habitats,  and  their  assumption  of 
practically  every  mode  of  life  exhibited  by  plants,  the  angiosperms 
stand  at  the  apex  of  the  vegetable  kingdom.  They  are  a  modern 
group  and  have  arisen  in  comparatively  recent  geological  time. 
Before  the  competition  of  this  new,  vigorous,  and  well-equipped 
phalanx,  the  older  vascular  plants  have  been  swept  aside,  most  of 
them  to  complete  extinction,  and  the  rest,  with  few  exceptions, 
to  comparative  insignificance.  It  is  only  the  thallophytes,  with 
their  specialization  for  aquatic,  parasitic,  and  saprophytic  habits 
of  life,  that  can  compare  with  angiosperms  in  number  of  species 
and  individuals,  and  we  must  remember  that  were  it  not  for  these 
higher  seed  plants,  practically  all  saprophytes  and  parasites  would 
perish.  The  angiosperms  are  of  primary  importance  as  food 
producers  for  animals  and  man. 

Since  it  is  the  members  of  this  group  which  we  have  studied 
almost  exclusively  in  the  earlier  chapters  of  the  text,  it  will  not  be 
necessary  to  treat  them  here  with  as  great  detail  as  we  have  the 
other  branches  of  the  plant  kingdom.  It  will  be  worth  while, 
however,  to  bring  together  the  essential  features  of  the  class  as  a 
whole,  that  we  may  readily  compare  it  with  the  other  seed  plants 
and  see  it  in  its  proper  relation  to  the  rest  of  the  plant  kingdom. 

Vegetative  Structures. — The  vegetative  body  is  much  diversified. 
Seed  plants  not  only  include  trees  and  smaller  woody  plants,  (the 
only  growth  forms  displayed  by  gymnosperms),  but  they  have 
developed  a  new  type,  the  herb,  which  is  particularly  well  adapted 
to  temperate  or  semi-arid  regions,  since  it  is  small  and  soft- 
stemmed,  produces  flowers  and  fruit  after  a  very  short  growing 
period,  and  can  survive  winter,  dry  seasons  and  other  unfavorable 
periods  either  underground   or  in   Ihe  form   of  resistant   seeds. 


356 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


In  perennial  herbs  the  underground  parts  survive  and  it  is  only 
the  upper  portions  which  die  back;  in  biennial  herbs,  the  plant 
lives  through  two  seasons,  storing  up  food  the  first  and  flowering 
the  second,  and  in  annual  herbs  the  plant  body  lives  through 
only  one  season,  and  survives  unfavorable  conditions  in  the  seed. 
Herbs  are  now  very  rich  in  species  and  include  the  majority  of 
our   food    plants   and    many   others    of   economic    importance. 


4nthGr 
-Filament 


Sepal 


Petal         Stamen 


(f 


Pisti 


-S+igma 
-Style 

-  Ovary 


-Chambers 
of  Ovary 

■  Ovule 


Cross  Scciior, 
of  Ovary 


Fig.  227. — The  structure  of  the  flower  of  a  dicotyledonous  seed-plant  (dia- 
grammatic). A,  face  view  of  the  flower,  showing  its  calyx  of  five  sepals,  its 
corolla  of  five  petals,  its  ten  stamens  and  its  pistil.  B,  longitudinal  section, 
showing  the  relations  between  the  parts.  1,  Receptacle.  2,  Calyx.  3,  Corolla. 
4,  Stamen.     5,  Pistil,  with  ovary  cut  lengthwise. 


Internally,  the  vascular  system  reaches  in  angiosperms  its 
highest  degree  of  specialization.  A  cambium  is  well  developed 
in  the  woody  members  but  is  much  reduced  among  herbs.  The 
wood  consists  not  only  of  those  general-utility  elements,  the 
tracheids,  but  also  of  thick-walled  fibers  whose  function  is  to 
furnish  rigidity  to  the  stem;  and  of  wide,  thin-walled  ducts  or 
vessels,  by  which  large  quantities  of  water  can  be  conveyed  rapidly 
through  the  wood.  In  the  bast,  too,  a  new  element,  the  com- 
panion-cell, intimately  related  to  the  sieve-tube,  makes  its 
appearance. 


THE  SPERMATOPHYTA 


357 


Reproduction. — The  gyinnospenns  are  all  wiud-pollinated,  and 
many  of  the  lower  angiospcrms  resemble  them  in  this  respect  and 
have  inconspicuous,  cone-like  reproductive  organs  (Fig.  107)  often 
not  differing  remarkably  in  general  appearance  and  function 
from  the  gymnosperm  type,  except  in  the  possession  of  ovaries. 


C  D 

Fig.  228. — The  process  of  seed-production  in  a  flowering  plant.  Longitudinal 
diagrams  of  flower  and  fruit,  the  calyx  and  corolla  solid  black;  the  ovule,  seed- 
coats  and  embryo  dotted,  and  the  ovary  wall,  style  and  stigma  lined.  A,  young 
bud,  the  stamens  and  the  single  ovule  beginning  to  develop.  B,  bud  ready  to 
unfold.  The  embryo-sac  within  the  ovule  is  fully  developed  and  the  egg  (below) 
and  double  endosperm  nucleus  (in  center)  are  ready  for  fertilization.  C,  fully 
opened  flower.  The  anthers  have  burst  and  pollination  has  taken  place,  pollen 
grains  being  transferred  to  the  stigma.  Two  grains  have  germinated,  and  the 
pollen-tube  from  one  of  them  has  penetrated  the  style,  entered  the  ovary,  passed 
through  the  micropyle  of  the  ovule  and  discharged  its  contents — the  two  male 
gametes — into  the  embryo-sac.  Double  fertilization  is  taking  place,  one  male 
gamete  uniting  with  the  egg  and  the  other  with  the  endosperm  nucleus.  D, 
ripe  fruit.  Sepals,  petals  and  stamens  have  dropped  off;  the  ovary  wall  has 
hardened  into  the  pericarp;  the  micropyle  has  closed;  the  integuments  have 
become  seed  coats  and  the  ovule  has  developed  into  the  seed.  The  embryo,  in 
the  center  of  the  seed,  has  grown  from  the  fertilized  egg,  and  the  endosperm 
surrounding  it  (shown  in  white)  from  the  endosperm  nucleus. 

The  higher  members,  however,  have  come  to  depend  upon 
insects  to  transport  their  pollen,  and  have  evolved  the  charac- 
teristic flower  (Fig.  227)  which  we  have  described  in  a  previous 
chapter,  with  its  protective  calyx,  composed  of  sepals;  its  attrac- 


358  BOTANY:  PRINCIPLES  AND  PROBLEMS 


Fig.  229. — Development  of  the  female  gametophyte  and  seed  in  a  dicotyledon- 
ous angiosperm.  Longitudinal  sections  through  the  ovule  and  seed.  A,  very 
young  ovule,  the  two  integuments  (1  and  2)  beginning  to  appear  and  the  mega- 
spore  mother-cell  evident  within  the  nucellus  (3).  B,  the  megaspore  mother-cell 
has  produced  a  row  of  four  megaspores.  C,  three  megaspores  have  aborted,  the 
fourth  is  enlarging.  D,  the  megaspore  nucleus  has  divided  into  two,  which  now 
lie  at  opposite  ends  of  the  young  embryo-sac  (4).  E,  each  nucleus  has  again 
divided  into  two.  F,  each  of  the  four  nuclei  has  again  divided  into  two,  so  that 
there  are  now  two  groups  of  four  nuclei,  one  at  each  end  of  the  embryo-sac. 
G,  one  nucleus  from  each  group  has  migrated  to  the  center,  and  the  two  are 
uniting  to  form  the  endosperm  nucleus  (6) .  The  three  left  at  the  end  of  the  sac 
next  the  micropyle  have  formed  the  egg-cell  (5)  and  the  two  synergid  cells.  The 
three  at  the  opposite  end  have  formed  the  antipodal  cells  (7) .  The  embryo  sac  is 
now  fully  developed  and  the  egg  is  ready  for  fertilization.  H,  a  pollen  tube 
has  entered  the  micropyle  and  discharged  two  male  cells  into  the  embryo  sac. 
One  of  these  (8)  is  uniting  with  the  egg  and  the  other  (9)  with  the  endosperm 
nucleus.  /,  the  fertilized  egg  has  grown  into  a  small  embryo  (10)  and  the  fertil- 
ized endosperm  nucleus  into  a  sac  lined  with  nuclei  (11).  /,  the  mature  seed. 
The  integuments  have  hardened  into  seed  coats,  the  micropyle  is  closed,  the 
nucellus  has  disappeared,  the  endosperm  sac  has  become  a  mass  of  solid  endo- 
sperm, packed  with  food,  and  the  embryo  has  reached  its  full  size. 


TIIR  SPERM  A  TOPJI  VTA 


359 


tive  corolla,  composed  of  petals;  its  pollen-producing  stamens,  and 
its  ovule-bearing  pistil.  The  pistil  may  be  a  single  carpel  (which 
has  grown  about  the  ovules  and  enclosed  them);  a  number  of 


/-^ 


\r  J 


•   • 


I  m  f' 


•• 


Fig.  230. — Development  of  the  embryo  sac,  and  fertilization,  in  the  squash. 

A,  megaspore  mother-cell,  which  is  situated  in  the  middle  of  a  very  young  ovule. 

B,  the  four  spores  which  develop  from  the  mother-cell.  Three  of  these  degener- 
ate and  disappear  but  the  fourth,  larger  than  the  others,  is  the  functional  mega- 
spore. C,  the  enlarged  megaspore,  in  which  the  nucleus  has  now  divided  into 
two.  D,  a  stage  slightly  later  than  C  The  two  nuclei  have  migrated  to  opposite 
ends  of  the  young  embryo-sac.  E,  a  still  later  stage,  with  the  embryo-sac  much 
increased  in  size.  F,  each  of  the  two  nuclei  has  divided  into  two.  G,  each  of 
these  four  nuclei  has  again  divided  into  two,  producing  eight  nuclei  in  all.  H, 
the  embryo-sac,  nearly  mature.  At  the  upper  end  (away  from  the  micropyle  of  the 
ovule)  are  three  antipodal  cells.  At  the  lower  end  (toward  the  micropyle)are  the 
egg  nucleus  and  the  two  synergids.  The  two  endosperm  nuclei  in  the  middle 
have  not  yet  fused.  I,  fertilization.  One  small  male  nucleus  is  about  to  unite 
with  the  endosperm  nucleus  (still  double)  and  the  other  is  about  to  fertilize  the 
the  egg.      {From  A.  I.  Wcinstcin). 


separate  carp(>ls;  or  a  group  of  carpels  which  have  become  fused 
together.     Tn   any  case,  the  pollen  is  necessarily  prevented  from 


360  BOTANY:  PRINCIPLES  AND  PROBLEMS 

alighting  directly  upon  the  micropyle  as  it  does  in  the  gymno- 
sperms,  but  instead  is  received  upon  a  special  projection  of  the 
pistil,  the  stigma.  Here  it  germinates  and  sends  down  a  pollen 
tube  which  ultimately  reaches  an  ovule  (Fig.  228). 

The  male  gametophyte  has  practically  disappeared,  for  the 
nucleus  of  the  microspore  now  divides  only  into  the  tube  nucleus 
and  the  generative  cell,  and  the  latter  produces  two  non-motile 
male  gametes.  The  female  gametophyte  is  also  greatly  reduced 
(Figs.  229  and  230).  The  megaspore  (the  successful  one  of  four 
originally  produced  in  the  nucellus)  becomes  much  enlarged. 
Its  nucleus  divides  into  two,  and  one  of  these  migrates  to  the 
end  of  the  young  embryo-sac  next  the  micropyle  of  the  ovule  and 
the  other  to  the  basal  or  antipodal  end.  Here  each  nucleus 
undergoes  two  further  divisions,  so  that  at  each  end  of  the  sac 
there  are  now  four  nuclei.  One  from  each  set  then  moves  toward 
the  middle  and  these  two  there  fuse  to  iormtheeridosperm nucleus. 
The  three  remaining  at  the  micropylar  end  of  the  sac  become 
definite  cells,  one  of  them  the  egg  cell  and  the  other  two  the 
synergids,  probably  the  remains  of  an  archegonium.  The  three 
antipodal  cells  represent  all  that  remains  of  the  abundant  vegeta- 
tive tissue  of  the  sac  as  it  is  found  in  the  gymnosperms.  The 
gametophyte  now  consists  of  seven  cells  (or  six  cells  and  a  naked 
nucleus)  and  is  ready  for  fertilization.  The  two  male  cells  pass 
down  the  pollen  tube  and  enter  the  ovule.  One  fertihzes  the  egg 
nucleus,  as  usual,  and  from  this  union  the  embryo  results. 
The  other,  however,  instead  of  being  eliminated,  unites  with  the 
endosperm  nucleus,  and  from  the  union  of  these  two  nuclei  (one 
of  which,  it  will  be  remembered,  is  already  a  product  of  fusion) 
develops  the  endosperm  of  the  seed.  The  endosperm  thus  differs 
markedly  in  its  history  from  that  of  gymnosperms.  This  phe- 
nomenon of  double  fertilization  is  a  distinctive  feature  of  reproduc- 
tion in  all  angiosperms.  The  development  of  the  embryo  and 
the  ripening  of  the  seed  go  on  much  as  among  the  lower  seed 
plants. 

The  angiosperms  are  divided  into  two  clearly  marked  subdivi- 
sions, the  dicotyledons  and  the  monocotyledons. 

Dicotyledoneae  or  Dicotyledons. — As  their  name  implies, 
these  plants  are  characterized  by  the  presence  of  two  cotyledons 
in  the  embryo  of  the  seed;  as  opposed  to  the  single  one  of 
monocotyledons,  and  they  also  display  several  other  distinctive 
features   (Figs.   231  and   232).   These  are:  (1)  The  presence  of 


THE  SPERMATOPHYTA 


361 


ncttod  venation  in  tho  leaf  as  opposed  to  the  typically 
parallel-veined  system  of  the  monocotyledonous  leaf;  (2)  the 
distribution  of  the  vascular  system  in  a  ring  or  tube  separat- 
ing an  internal  pith  from  an  external  cortex,  as  opposed  to 
the  irregularly  scattered  system  of  separate  bundles  in  the 
monocotyledonous  stem;  and  (3)  the  development  of  the  floral 


Fig.   231. 

Fig.  231. — Characteristics  of  a  typical  dicotyledonous  plant.  .1,  loaf, 
netted-veined.  B,  .stem,  showing  ring  of  wood  and  bast.  C  and  D,  the  flower, 
showing  the  floral  parts  in  fives.  E,  section  of  seed,  showing  dicotyledonous 
embryo.  (From  Gager's  "Fundamentals  of  Botany",  P.  Blakiston's  Son  and  Co., 
Philadelphia). 

Fig.  232. — Characteristics  of  a  typical  monocotyledonous  plant.  A,  leaf, 
parallel-veined.  B,  stem,  showing  fibro-vascular  bundles  irregularly  scattered. 
C  and  D,  the  flower,  showing  the  floral  parts  in  multiples  of  three.  E,  section  of 
seed,  showing  monocotyledonous  embryo.  (From  Gager's  "Fundamentals  of 
Botany",  P.  Blakiston's  Son  and  Co.,  Philadelphia). 


parts  in  multiples  of  four  or  five  as  opposed  to  the  construction  of 
the  monocotyledonous  flower,  which  is  typically  on  the  plan  of 
three. 

The  dicotyleilons,  which  include  over  100,000  species,  are 
generally  divitled  into  two  main  groups;  the  more  primitive 
Archirhldvnjdeae,  in  which  the  calyx  and  corolla  ni-e  either  vei'v 


302 


BOTANY:  PRINCIPLES  AND  PROBLEMS 


poorl}^  developed,  or  have  their  various  members  entirely  separate 
from  one  another,  and  the  Syrnpetalae,  in  which  the  petals  arc 
tj^pically  united  into  a  gamojietalous  corolla. 

Of  the  great  array  of  groups  which  compose  the  dicotyledons 
we  shall  mention  only  the  most  important.  The  Archichlamy- 
deae  include,  among  others,  the  following  orders: 

Amentiferae. — This  group  is  now  commonly  divided  into  a 
number  of  subordinate  orders,  but  in  its  larger  sense  it  includes 
the  oaks,  beeches,  chestnuts,  hickories,  walnuts,  birches,  alders. 


m 

:VlH^^H 

Fig.  2.33.— One  of  the  Ranales. 
Hepatica  (Hepatica  triloba),  belonging 
to  the  family  Ranunculaceae. 


Fig.     _':il         ■  the     Resales. 

Choke     cherrj-      irruutis     virginiana) , 
belonging  to  the  family  Rosaceae. 


willows,  poplars,  and  many  others,  all  of  them  trees  or  shrubs. 
The  perianth  is  scale-like  or  absent,  the  flowers  dry  and  chaffy 
and  arranged  in  a  long,  sometimes  cone-like  inflorescence,  the 
ament  or  catkin  (Fig.  107).  They  are  prevailingly  wind-polli- 
nated. These  plants  were  long  regarded  as  the  most  ancient  of 
the  dicotyledons,  but  are  now  looked  upon  by  many  botanists 
as  simplified  and  reduced  forms. 

Ranales  (Fig.  233). — This  great  and  varied  group  contains  the 
buttercup,  magnolia,  laurel,  and  water-lily  families  and  their 
allies,  including  trees,  shrubs,  and  herbs.  Its  flowers  are  polli- 
nated by  insects  and  are  primitive  in  the  sense  that  their  parts, 


77/ 1<:  Sl'KUMA  TOP  1 1 ) '  TA 


363 


particularly  the  stamens  and  carpels,  arc  typically  numerous 
and  have  not  become  stereotyped  in  number  and  arrangement  as 
is  often  the  case  among  higher  orders.  The  Ranales  are  regarded 
by  many  as  the  most  ancient  of  the  dicotyledons  and  as  the  center 
of  origin  for  many  of  the  higher  groups. 

Rosales  (Fig.  234). — This  huge  order  includes  the  saxifrage,  rose, 
and  legume  families  and  their  allies,  comprising  trees,  shrubs,  and 
herbs.  The  flowers  are  for  the  most  part  conspicuous  and 
attractive,  and  furnish  a  large  number  of  our  ornamental  plants. 
Many  important  fruits  and  vegetables  also  find  their  place  here. 


Fig.  235.— One  of  the  Umbellales.  Fig.  236.— One  of  the  Rubiales. 
Wild  carrot  {Daucus  carota) ,  belonging  Button-bush  (Cephalanthus  occiden- 
to  the  family  Umbelliferae.  talis),  belonging    to    the    family  Rubi- 


Kegularity  and  symmetry  characterize  the  flowers  of  the  lower 
members  but  in  the  legume  family  (Leguminosae)  the  corolla 
becomes  markedly  irregular,  producing  the  butterfly-like  or 
papilionaceous  type. 

U^nbeUales  (Fig.  235).— This  includes  the  dogwood  family 
(mostly  shrubs)  and  the  carrot  family  (mostly  herbs).  The 
ovary  in  the  group  is  inferior,  the  other  floral  parts  being  fused 
with  it  or  borne  upon  it.  The  flowers  are  very  small  and 
arranged  in  compact,  flat-topped  clusters  called  umbels.     This 


364  BOTANY:  PRINCIPLES  AND  PROBLEMS 

order  marks  the  highest  point  of  development  among  the 
Archichlamydeae. 

The  following  orders  are  most  important  among  the  Sympetalae : 

Ericales  (Fig.  109). — These  are  the  heaths,  mountain  laurels, 
blueberries,  and  their  allies,  most  of  them  shrubby  plants, 
including  many  evergreens.  The  order  is  intermediate,  in 
certain  of  its  characters,  between  Archichlamydeae  and 
Sympetalae. 

Tubiflorales  (Fig.  104). — Here  are  placed  the  morning-glory, 
phlox,  borage,  verbena,  nightshade,  mint,  and  figwort  families 
and  a  number  of  others,  most  of  the  members  of  which  are  herbs. 
The  corollas  are  conspicuous  and  prevailingly  tubular,  and  are 
regular  in  the  lower  families  but  extremely  irregular  in  the  higher 
ones.  The  order  comprises  not  only  many  of  our  garden  flowers 
but  a  number  of  such  important  crop  plants  as  the  potato  and 
tobacco. 

Ruhiales  (Fig.  236). — Here  are  the  madder  and  honeysuckle 
families,  which  are  prevailingly  trees  and  shrubs,  although 
herbaceous  species  also  occur  among  them.  The  flowers  are 
most  commonly  on  the  plan  of  four  and  tend  to  be  grouped  in 
compact  clusters. 

Campanulales  (Fig.  239). — This  order  includes  the  gourd, 
bellwort,  and  composite  families,  most  of  the  members  of  which 
are  herbaceous.  The  last  is  the  largest  and  most  widespread 
family  among  angiosperms.  Its  flowers  are  arranged  in  complex 
heads,  each  of  which  somewhat  resembles  a  single  flower,  the 
flowers  at  the  margin  of  the  head  often  being  different  from 
those  in  the  center.  The  calyx  is  greatly  reduced  and  becomes  a 
scalelike  pappus  surmounting  a  single-seeded  ovary.  This 
family  is  a  remarkably  successful  one  and  marks  the  highest  point 
in  evolutionary  advance  among  the  dicotyledons. 

Monocotyledoneae  or  Monocotyledons. — These  plants,  of 
which  there  are  some  30,000  species,  are  characterized  by  the 
possession  of  a  single  cotyledon  in  the  embryo,  closed  or  parallel 
venation  in  the  leaf,  scattered  vascular  bundles  in  the  stem,  and 
a  floral  plan  based  on  multiples  of  three.  The  group  probably 
arose  from  somewhere  among  the  primitive  dicotyledons  and 
its  evolutionary  advance  has  been  more  or  less  parallel  with 
that  of  the  latter  group.  With  the  exception  of  one  order, 
monocotyledons  are  all  herbs.  Among  the  important  orders 
are  the  following: 


THE  SPERM  A  TOPH  YTA 


365 


Glumales  (Fig.  237). — These  are  the  grasses  and  the  sedges. 
The  small  flowers  lack  a  typical  calyx  and  corolla,  are  protected 
hy  chaffy  bracts,  and  are  arranged  in  clusters.  They  are  wind- 
pollinated,  except  for  those  cases  where  pollination  is  affected 
directly,  without  external  agency.  As  in  the  Amentiferae,  their 
simple  condition  may  have  come  about  through  reduction.  The 
grass  family  includes  the  most  important  of  our  crop  plants. 


Fig.  237.— One  of  the  Glumales. 
A  sedge  (Carex),  belonging  to  the 
family  Cyperaceae. 


Fig.  238.— One  of  the  Arales.  The 
skunk  cabbage  (Symplocarpus  foetidus) , 
belonging  to  the  family  Araceae. 


Palmales. — The  palms  are  a  tropical  tree-like  family  in  which 
the  columnar  trunk  is  surmounted  by  a  cluster  of  large  leaves. 
The  small  flowers  are  borne  in  spikes  and  in  some  species  are 
pollinated  by  wind  and  in  others  by  insects.  The  perianth  is 
very  simple. 

Arales  (Fig.  238). — These  are  the  aroids,  a  family  of  large  herl)s 
particularly  abundant  in  the  tropics  and  represented  with  us  only 
by  the  Jack-in-the-pulpit,  skunk's  cabbage,  and  a  few  other 
species.  The  leaves  are  typically  large,  and,  unlike  those  of  most 
monocotyledons,  are  netted-veined.  The  very  simple  flowers, 
almost  devoid  of  a  perianth,  are  clustered  on  a  fleshy  spadix 
which  is  enveloped  by  a  large  and  often  brilliantly  colored  bract, 
the  spathe. 


366  BOTANY:  PRINCIPLES  AND  PROBLEMS 

Liliales  (Fig.  98). — These  include  the  Hly,  amarylhs,  and  iris 
famihes,  perennial  herbs  with  well  developed  bulbs  or  rootstocks. 
The  species  usually  have  a  conspicuous  perianth  and  provide 
some  of  our  most  beautiful  garden  flowers. 

Orchidales  (Fig.  240). — The  orchids  are  notable  for  the  extreme 
irregularity  and  great  beauty  of  their  flowers,  and  their  remark- 


FiG.  239.— One  of  the  Campanuhilos.  Fig.  240. — One   of   the   Orchidales. 

Yellow  chamomile  (Anthemis  tinctoria),      Showy    lady's     slipper    {Cypripedium 
belonging  to  the  family  Compositae.  hirsutum),    belonging    to     the    family 

Orchidaceae. 


able  specializations  for  insect  pollination.  They  are  particularly 
abundant  and  brilliant  in  the  tropics,  whence  they  have  been 
introduced  into  the  greenhouses  of  cooler  chmates  as  the  most 
highly  prized  of  exotics. 

With  the  development  of  the  composite  and  orchid  families, 
the  plant  kingdom  has  reached  the  climax  of  its  evolution. 

QUESTIONS  FOR  THOUGHT  AND  DISCUSSION 

790.  Why  have  plants  which  reproduce  by  seeds  rather  than  those 
which  reproduce  by  spores  become  the  most  successful  part  of  the  world's 
vegetation? 


THE  SPERMATOPIIYTA  367 

791.  Can  you  suggest  why  seed  plants  have  so  very  many  more 
species  than  either  bryophytes  or  pteridophytes? 

792.  From  a  consideration  of  the  present  distribution  over  the  earth 
of  ferns  and  of  primitive  seed  plants,  what  suggestion  can  you  make  as 
to  the  climatic  conditions  under  which  the  earliest  seed  plants  were 
evolved  ? 

793.  Why  do  you  think  it  is  that  seed  j)lants  are  not  very  common  in 
the  ocean? 

794.  The  name  "Flowering  Plants"  is  sometimes  applied  to  the  seed 
plants.     Why  is  it  not  a  very  satisfactory  name? 

795.  The  archegonium  is  much  reduced  in  the  gametophyte  of  the 
seed  plants.     Explain. 

796.  With  what  in  the  ferns  is  the  embryo-sac  of  seed  plants  compara- 
ble? the  pollen  grain?  the  ovule?  the  nucellus? 

797.  To  what  in  seed  plants  do  the  following  structures  in  liverworts 
correspond:  The  spores;  the  sporangium  wall  and  seta;  the  thallus;  the 
sperms? 

798.  Different  terms  are  used  for  the  various  structures  in  the  plant's 
life  history  among  seed  plants  than  are  used  among  bryophytes  and 
pteridophytes  (embryo-sac  for  female  gametophyte,  anther  for  sporan- 
gium, and  so  on).     What  reason  can  you  suggest  for  this? 

799.  What  great  groups  of  animals  are  there  which  obtain  the  bulk 
of  their  food  directly  or  indirectly  from  other  plant  groups  than  the  seed 
plants? 

800.  What  similarities  can  you  suggest  in  the  evolutionary  history  of 
bryophA^tes,  pteridophytes,  and  spermatophytes,  in  the  plant  kingdom, 
to  that  of  amphibians,  reptiles,  and  mammals  in  the  animal  kingdom? 

801.  What  various  resemblances  do  the  cycads  show  to  the  ferns? 

802.  In  what  way  is  the  relation  of  the  conifers  to  the  rest  of  the 
gymnosperms  similar  to  that  of  the  ferns  to  the  rest  of  the  pteridophytes? 

803.  What  relation  can  you  suggest  between  the  characteristic  wood 
structure  of  the  conifers  and  the  tough  and  scale-like  character  of  tlieir 
leaves? 

804.  In  such  a  tree  as  the  pine,  are  the  staminate  or  the  ovulate 
cones  more  numerous?     Explain. 

805.  In  regions  where  there  are  large  coniferous  forests,  "showers  of 
sulphur"  sometimes  occur  in  the  spring.  What  explanation  can  you 
suggest  for  these? 


368  BOTANY:  PRINCIPLES  AND  PROBLEMS 

806.  What  advantages  have  the  angiosperms  over  the  gymnosperms 
which  should  have  made  them  so  much  more  successful? 

807.  Give  an  example  of  a  crop  plant  which  is  an  annual  herb;  one 
which  is  a  biennial  herb;  one  which  is  a  perennial  herb;  one  which  is  a 
shrub;  one  which  is  a  tree. 

808.  Which  of  these  five  plant  types  is  of  most  importance  in  agricul- 
ture?    Why? 

809.  There  is  a  larger  proportion  of  herbaceous  species  in  the  flora  of 
a  temperate  region  than  in  the  flora  of  a  tropical  one.     Explain. 

810.  Angiosperms,  and  especially  herbaceous  angiosperms,  have 
apparently  evolved  much  faster  and  given  rise  to  many  more  species 
than  either  pteridophytes  or  gymnosperms  in  a  similar  length  of  time. 
Why? 

811.  What  evidence  can  you  suggest  for  the  conclusion  that  the 
dicotyledons  are  more  ancient  than  the  monocotyledons? 

812.  We  believe  that  families  in  which  the  flowers  are  irregular  are 
usually  more  recent  in  evolutionary  origin  than  those  where  the  flowers 
are  regular.     Why? 

813.  We  believe  that  a  flower  with  a  large  and  indefinite  number  of 
floral  parts,  all  free  from  one  another,  is  more  ancient  in  type  than  one 
in  which  the  number  of  parts  is  smaller  and  more  constant  and  where 
they  are  more  or  less  united  with  one  another.     Why? 

814.  In  what  respect  may  stamens  and  carpels  be  called  "sexual 
organs"  and  in  what  respect  is  it  incorrect  to  call  them  such? 

815.  What  conclusion  can  you  draw  as  to  the  relative  osmotic  concen- 
tration of  the  cell  sap  in  the  pollen  grains  and  in  the  cells  of  the  stigma? 

816.  Why  do  we  regard  the  composite  and  the  orchid  families  as 
marking  the  highest  points  in  the  evolution  of  the  plant  kingdom? 

REFERENCE  PROBLEMS 

138.  What  familj'  is  most  important  as  a  producer  of  food  plants  and  what 
are  some  of  the  notable  crop  plants  included  in  it?  Name  a  few  other 
families  which  have  many  important  economic  plants. 

139.  Seed  plants  are  sometimes  known  as  the  Siphonogamia.  Whj'  is  this 
name  appropriate  for  them? 

140.  The  terms  Cryptogams  and  Phanerogams  are  sometimes  used  to 
designate  two  groups  into  which  the  plant  kingdom  may  be  divided,  the 


THE  SPERM ATOPH VTA  369 

former  including  thallophytes,  bryophytes,  and  pteridophytes,  and  the 
latter  including  seed  plants.  Why  is  such  a  classification  not  a  very  good 
one? 

141.  Name  five  cultivated  plants  which  belong  to  the  rose  family;  five 
which  belong  to  the  nightshade  family;  five  which  belong  to  the  composite 
family, 

142.  What  family  of  plants  is  most  important  as  far  as  the  production  of 
cultivated  fruits  is  concerned? 

143.  What  family  of  plants  is  most  important  as  far  as  the  production  of 
timber  is  concerned? 

144.  Give  the  derivation  of  the  following  terms  and  explain  in  what  way 
each  is  appropriate: 

Antipodal  Cells  Synergid  Papilionaceous 


INDEX 


Figures  in  t)()ldf:ice  type  indicate  page8  on  which  ilhistrations  occur. 


Abies,  15 

Absorption,  of  water  and  salts,  53,  54 

bands,  in  spectrum,  72 
Accommodation,    to    unusual    tem- 
peratures, 157 
Acer,  6  species  of,  258 
Achene,  200 
Acquired  characters,  inheritance  of, 

210,  235 
Actnea,  fruit  of,  201 
Adaptation,  155 
Adder's  tongues,  333 
Adiantum,  stem  of,  329 
Aecidiospore,  298 
Aecidium,  298 
Agaricaceae,  300 
Agaricus,  299 
Agriculture,  3 
Air,  in  soil,  29 
Albugo,  292 
Alcohol,  produced  by  fermentation, 

131 
Aleurone-grains,  122 
Aleurone  layer,  122 
Alga-like  fungi,  289 
Algae,  11,  12,  264-284 

blue-green,  265,  266 

brown,  278 

green,  266 

in  lichens,  303,  304 

red,  282 
Allelomorph,  213 
Alternate  leaf  arrangement,  90 
Alternation  of  generations,  248 

advantages  of,  249 

in  bryopliytes,   250,   311 

in  pteridophytes,  250 


Alternation  of    generations,    in  red 
algae,  284 

in  spermatophytes,  251 

in  thallophytes,  249 

origin  of,  249 
Anient,  362 
Amentiferae,  362 
Amino-acids,  122 
"Amphibious"  plant,  164 
Anaphase,  of  mitosis,  140 
Anatomy,  3 

Anemophilous  flowers,  194 
Angiosperms,  15,  16,  355-360 

female  gametophyte  of,  358,  359 
Animals,  compared  with  plants,  244 
Annual  ring,  101,  148 
Annulus,  329,  331 
Anthemis,  366 
Anther,  185 

Antheridial  filament,  291 
Antheridium,  271,  312,  321 
Anthoceros,  317,  318 
Anthocerotales,  317 
Antipodal  cells,  358,  359 
Antitoxin,  288 
Apophysis,  322 
Apple,  leaf  of,  64 
Arales,  365 

Archegonium,  312,  321 
Archichlamydeae,  361 
Arctium,  fruit  of,  201 
Aristotle,  4 
Ascent  of  sap,  111 
Asclepias,  fruit  of,  200 
Ascocarp,  293 
Ascogonium,  295 
Ascomycetos,  292 
Ascosporo,  293 
Ascus,  293 

Asexual  reproduction,  182 
Ash,  of  plants,  31 


371 


372 


INDEX 


Aspergillus,  295 
Aspidium,  14,  328 
Assimilation,  20,  124 
Autobasidioraj-cetes,  299 
Auxiliary  cell,  284 
.\xil,  of  leaf,  90 

B 

Bacillus,  285,  286 
Bacteria,  285-289 

in  soil,  32 

nitrifying,  32,  33,  289 

nitrogen-fixing,  32,  33,  289 

pathogenic,  288 

saprophytic,  287 
Bacterium,  286 
Bacterium,  32 
Baneberry,  fruit  of,  201 
Barberry,  rust  of,  298 
Bark,  91,  94,  96 
Basidia  fungi,  296 
Basidiomycetes,  296 
Basidiospore,  296 
Basidium,  296 
Bast,  97-100 

of  root,  47 
Bast-fiber,  97,  98 
Bean,  etiolated  seedling  of,  160 

germination  of,  202 

seed  of,  199 

starch  grains  of,  120 
"Bee  bread,"  194 
Beet  sugar,  119 
Bennettitales,  350 
Berry,  200 
Binomial  s.ystem   of  nomenclature, 

260 
Black  fungi,  294 
B  ack  knot,  of  plums,  294,  295 
Black  molds,  290 
Blade,  of  leaf,  63,  64,  65 
Blights,  289,  291 
Blueberry,  flower  of,  190 
Blue-green  algae,  265,  266 
Blue  molds,  295 
Body-cell,  of  pollen,  347,  353 
Boston  fern,  bud  mutation  in,  223 


Botany,  defined,  1 

history  of,  4-8 

importance  of,  1 

subdivisions  of,  2-4 
Branch,  89 
Brown  algae,  278 
Bryales,  320 

Bryophyta,  11,  14,  311-323 
Bud,  88,  92 

lateral,  88,  92 

scales,  89 

terminal,  88,  92 
Budding,  of  yeasts,  295,  296 

reproduction  by,    183,   295 
Bulb,  93,  95,  184 
Bundle  sheath,  67,  104 
Buphthalmum,  pollen  of,  186 
Burdock,  fruit  of,  201 
Butter,  spoiling  of,  133 
Button-bush,  flower  of,  363 


Calcium,  importance  of,  31 
Calyptra,  321,  322 
Calyx,  188 
Cambium,  101,  146-149 

cork,  148 

interfascicular,  104 
Campanulales,  364 
Cane  sugar,  119 
Capillarity,  25-29 

in  sap  ascent.  111 
Capillary  water,  in  soil,  25 
Capsule,  fruit  type,  199 

of  mosses,  321,  322 
Carbohydrates,  119-121 
Carbon  dioxide,  in  atmosphere,  69 

in  photosynthesis,  69 
Carex,  365 
Carotin,  70 
Carpel,  346 

Carpogonium,  282,  283 
Carpospore,  282,  283 
Carya,  91 

Catkin,  192,  193,  362 
Cell,  19,  44,  45,  100,  139 

division  of,  139,  140,  144 


INDEX 


373 


Cell,  enlargement  of,  141,  144 

maturation  of,  142,  144 

plate,  141 

size,  constancy  of,  138 

theory,  6 

wall,  44,  45,  139 
Cells,  of  wood,  100,  101 

production  of  new,  139-142 
Cellulose,  45,  120,  124 
Centrifugal  force,  as  substitute  for 

gravity,  162 
Cephalanthus,  flower  of,  363 
Cercis,  living  and  fossil  species  of, 

230 
Cesalpino,  5 

Chamomile,  flower  of,  366 
Char  a,  277 
Charales,  277 

Chemical    substances,    as    environ- 
mental factors,  168 
Cherry,  flower  of,  189 

twigs  of,  93 
Chinese  primrose,  flower  of,  195 
Chlamydomonas,  247,  268 
Chlorophyceae,  266 
Chlorophyll,  19,  67,  70 
Chloroplast,  67,  68,  70 
Choke  cherry,  362 
Chromatin,  140 
Chromatophore,  267 
Chromosome,  139,  140 

number,  140 

reduction,  186,  187,  188,  217,  251 
Cilia,  267 

Circaea,  pollen  of,  186 
Classical  Period,  4 
Classification,  based  on  relationship, 
256 

natural,  257 

of  plants,  256 

units  of,  259 
Club  mosses,  334 
Cobaea,  pollen  of,  186 
Coccus,  285,  286 
Coenocyte,  272 
Cohesive  power,  of  water  column, 

112 
Coleochaete,  271,  272 


Columella,  in  RMzopus,  289 
Combustion,  compared  with  respira- 
tion, 126 
Companion-cell,  98,  99,  356 
Compound  leaf,  64 

ovary,  190 
Conceptacle,  of  Fncns,  280,  281 
Cone,  335,  347,  351,  352,  353,  354 
Confervales,  270 
Conferva-like  algae,  270 
Conidia,  292 
Coniferales,  351 
Conifers,  351 
Conjugales,  274 
Conjugating  tube,  274,  275 
Cork  cambium,  148 
Corky  bark,  94,  96 
Corm,  93,  95,  184 
Corn,  kernel  of,  199 

starch  grains  of,  120 

stem  of,  103 
Corolla,  187 
Cortex,  of  root,  43,  46 

of  stem,  95,  96 
Corydalis,  pollen  of,  186 
Corymb,  193 

Cotton-grass,  fruit  of,  200 
Cotyledons,  198,  199 
Crataegus,  16 
Crocus,  corm  of,  95 
Cross,  194 

Cross-fertilization,  198 
Cross-pollination,  194 
Crumbs,  of  soil,  24 
Crust,  on  soil  surface,  28 
Cube  of  wood,  in  three  planes,  108 
Cucurhita,  pollen  of,  186 
Cup  fungi,  293 
Cuscuta,  as  parasite,  171 
Cuticle,  65 

Cuttings,  reproduction  b\',  182 
Cyanophyceae,  265,  266 
Cycadales,  349 
Cycad-ferns,  349 
Cycadofilicales,  349 
Cycads,  349 
Cycas,  349,  350,  351,  352 

stonui  of,  165 


374 


INDEX 


Cyme,  193 
Cypripedium,  366 
Cystocarp,  282,  283 
Cytology,  3 
Cytoplasm,  45,  139 
Cytoplasmic  membrane,  52 


Double  fertilization,  358,  359,  360 
Downy  mildews,  291 
Drupe,  200 

Dry  weight,  definition  of,  82 
Duct,  101,  356 
in  vein,  68 


E 


Dandelion,  environmental  variation 
in,   210 

root  of,  41 
Darwin,  7,  235,  236 

theory  of,  235 
Daucus,  flower  of,  363 
Decay,    produced   by   bacteria,    32, 

133,  288 
Delphinium,  plant  of,  89 

stem  of,  103 
Desert  association,  170 
Desmids,  275 

Diageotropic  response,  162 
Diagrams  of  various  flower  types, 

189,  190 
Dianthus,  pollen  of,  186 
Diaphototropic  response,  158 
Diastase,  124 
Diatomin,  276 
Diatoms,  276 
Dicotyledons,  360,  361 

embryo  of,  198,  199 

flower  of,  185 
Differentiation,  during  growth,  149 

origin  of,  245 
Diffusion,  47,  48 

in  the  plant  cell,  51-53 
Digestion,  20,  122-124 
Digitalis,  stem  of,  103 
Dihybrid  cross,  220,  221 
Dimorphism,  of  flowers,  195 
Dioecious,  192,  342 
Dioscorides,  5 
Discomycetes,  294 
Division  of  cells,  140 

of  labor,  17,  246 
Dodder,  as  parasite,  171 
Dominance,  213 

incomplete,  213,  218 


Ecology,  3,  154 
Economic  botany,  3 
Ectocarpus,  278,  279 
Egg,  198,  247,  267 

fertilized,  198,  354 
Elaters,  316,  340,  342 
Elderberry,   variation  in  leaves  of, 

208 
Elements,  essential  for  plant  growth, 

31 
Embryo,  198,  199,  345,  348,  358 
Embryo-sac,  197,  346,  358,  359 
Empetrum,  leaf  blade  of,  165 
Endodermis,  of  root,  46 
Endosperm,  198,  199,  348,  358,  360 

nucleus,    197,   359,   360 
Energy,  kinetic,  69,  125 

potential,  69,  125 

relations  of  plants,  125 

release  of,  126 
Enlargement  of  young  cells,  141 
Environment,  154-177 

complexity  of,  177 
Enzymes,  123 
Epidermis,  of  leaf,  65,  66 

of  root,  46 

of  stem,  94 
Epigynous  floral  parts,  191 
Epipetalous  floral  parts,  191 
Epiphyte,  174,  175 
Episepalous  floral  parts,  191 
Equisetineae,  339 
Equisetum,  339,  340,  341 

gametophytes  of,  341 
Ericales,  364 
Eriophorum,  fruit  of,  200 
Etiolation,  159,  160 
Evidences  for  evolution,  229-235 

from  geographical  distribution,  231 


INDEX 


37i 


Evidences  for  evolution,  goological, 
230 

morphological,  231 

taxonomic,  231 
Evolution,  theory  of,  7,  229-239 


F 


Factor,  206 
"Family  tree,"  257 
Fats,  121 

digestion  of,  124 
Fatty  acids,  121 
"Female"  flower,  192 
Fermentation,  131 
Ferments,  123 
Ferns,  12,  14,  327-334 

gametophyte  of,  330 
Fertilization,  196-198,  345,  354 

double,  196 
Fertilized  egg,  198,  354 
Fibers,  of  wood,  356 
Fibro-vascular  bundles,  of  leaf,   68 

of  stem,  103,  104 
Fibro-vascular  cylinder,  of  root,  46 

of  stem,  95,  96 
Fibro-vascular  tissues,  326 
Filament,  185 
Filicales,  327 
Filicineae,  327 
Fir,  15 
Fission,  244 
Flagellates,  268 
Flocculation,  24 
Floccules,  in  soil,  24 
Floral  diagrams,  189,  190 
Flower,  20,  184-193,  348,  356,  357 

anemophilous,  194 

color  of,  191 

dimorphic,  195 

entomophilous,  194 

"female,"  192 

irregular,  191 

"male,"  192 

naked,  192 

regular,  191 

size  of,  191 

texture  of,  191 


Flower,  unisexual,  192 

variation  in,  191 
Jood,  69,  118 

as     a     storehouse     of     potential 
energy,  126 

compared  to  fuel,  126 

translocation  of,  112 
Foot,  315,  316,  318 
Fossils,  230 

Four-o'clock,  inheritance  in,  226 
Frond,  327 
Fructose,  119 
Fruit,  196,  199 

fleshy,  200,  201 

sugar,  119 
Fruiting  body,  of  ascomycetes,  293 
Fucales,  279 
Fucus,  12,  280 
Fungi,  11,  13,  264,  284-303 

alga-like,  289 

basidia,  296 

sac,  292 


Galls,  168,  169 

Gametes,  184,  206,  246,'248,  268 

Gametophyte,  248,  250,  284,  311 

of  seed  plants,  male,  347,  360 

of  angiosperms,  female,  358,  359 

of  ferns,  330 

of  gymnosperms,  female,  345 
Gamopetalous  corolla,  190 
Gamosepalous  calyx,  190 
Gaylussacia,  evolution  of  species  in, 

234 
Gene,  206 
Generative  cell,  197 
Generic  name,  260 
Genetics,  3,  210 
Genotype,  217 
Gentiann,  pollen  of,  186 
Genus,  258 

Geographical  distribution  and  evolu- 
tion, 231 
Geotropism,  162 

mechanism  of,  162 

negative,  162 

positive,  162 


376 


INDEX 


Germination  of  seed,  202 

Gill  fungi,  299,  300 

Gills,  of  fungi,  296,  300 

Ginkgo,  351 

Girdling,  effect  of,  112 

Gleba,  302 

Gliadin,  121 

Gloeocapsa,  265,  266 

Gloxinia,  starch  grains  of,  120 

Glucose,  72,  119 

Glumales,  365 

Glutamic  acid,  122 

Glycerine,  121 

Glycine,  122 

Gnetales,  354 

Grafting,  151 

reproduction  by,  183 
Grain,  199,  200 
"Grain"  of  wood,  107 
Grape  hyacinth,  growth  of,  176 

sugar,  72,  119 
Grass,  root  of,  40 
Gravity,  as  environmental  factor,  161 

as  stimulus,  161 
Green  algae,  266 

molds,  295 
Grew,  5 

Groups  within  groups,  258 
Growing-points,  142-149 

lateral,  146 

terminal,  143 
Growth,  138-150 

in  length,  143-146 

in  thickness,  146-149 

of  root-tip,  143,  144,  145 

of  stem-tip,  146 

rate  of,  176 

rings,  148 
Guard  cells,  of  stoma,  66 
Gymnosperms,  349-355 

gametophytes  of,  345,  347 
Gymnosporangium,  galls  formed  by. 


Hair-cap  moss,  14 
Hardiness,  differences  in,  158 


Haustorium,  172,  285 

Hawthorn,  16 

Head,  of  flowers,  193,  363 

Heart-wood,  105,  106 

Heat,  liberated  in  respiration,  157 

Helvellales,  294 

Hepatica,  362 

Hepaticae,  313 

Herb,  88,  89,  355 

origin  of,  102 

stem  of,  101,  103 
Herbalists,  5 
Hereditary  bridge,  207 
Heredity,  206-225 

versus  Environment,  177 
Heterocyst,  266 
Heterogamy,  269 
Heterosporous,  334 
Heterospory,  326 
Heterozygous,  217 
Hibiscus,  pollen  of,  186 
Hickory,  91 
Hilum,  of  seed,  199 

of  starch  grain,  120 
Histology,  3 
Holdfast,  270,  271 
Homosporous,  335 
Homozygous,  217 
Hormogonia,  266 
Horsechestnut,  twig  of,  92 
Horsetails,  339-342 
Host,  172 
Humus,  30 
Hyacinth,  bulb  of,  95 
Hydnaceae,  302 
Hydrodictyon,  269,  270 
Hydrolysis,  in  digestion,  123 
Hydrophyte,  166,  167 
Hydropteridales,  333 
Hydrostatic  water,  25 
Hydrotropism,  163 
Hygroscopic  water,  29 
Hymenium,  294 
Hymenoptera,  194 
Hypha,  285 
Hypocotyl,  198,  199 


INDEX 


377 


Imbibition,  51 
Immunity,  2SS 

Independent  assortment,  in  inheri- 
tance, 219 
Indian  Pipe,  173,  174 
Indusium,  328,  331 
Inflorescence,  193 
Ingenhousz,  6 
Inheritance,  206-225 

laws  of,  209 

Mendel's  law  of,  211-223 

of  acquired  characters,  210 
Insect-pollination,  194,  195 
Insects,  relation  of,  to  flowers,  194 
Insectivorous  plants,  173 
Integument,  197,  345,  346,  358 
Interfasicular  cambium,  104 
Internode,  90 
Intolerance,  of  shade,  161 
Invasion  of  the  land,  252 
Invertase,  123 
Iron,  in  formation  of  chloropliyll,  70 

importance  of,  31,  168 
7ns,  rootstock  of,  94 
Irregular  flower,  191 
Irritability,  of  protoplasm,  155 
Isoetes,  338,  339 
Isogamy,  268 
Isthmus,  of  desmids,  275 


Judas  tree,  living  and  fossil  species 

of,  230 
Jungermanniales,  317 

K 

Kalmia,  flower  of,  195 
Kelps,  278 
Kernel,  of  corn,  199 
Kinetic  energy,  69,  125 


Lady's  Slipper,  flower  of,  366 
Lamarck,  theory  of,  235 
Laminaria,  278 


Land,  difficulties  of  life  on,  252 

invasion  of,  252 

plants,  charucteri.stics  of,  253 
Larksjjur,  89 

Lateral  growing-point,  146 
Laws  of  inheritance,  209,  211 
Leaf,  19,  63-78,  325 

attachment  of,  90 

blade  of,  19,  63,  64,  65 

external  structure  of,  63,  64 

internal  structure  of,  65-69 

margin  of,  64 

petiole  of,  19,  63,  64 

simple  and  compound,  64 

stipules  of,  63,  64 

venation  of,  63,  65 
Leaf-gap,  101,  329 
Leaflets,  64 
Leaf-trace,  101,  329 
Leafy  liverworts,  317 
Leaves,  arrangement  of,  90 
Lenticel,  92 
Leucine,  122 
Leucoplast,  120 
Lichens,  175,  303,  304 

crustaceous,  303 

foliose,  303 

fruticose,  303 

symbiosis  in,  303,  304 

synthesis  of,  303 
Life  history  of  a  fern,  332 
Light,  as  environmental  factor,  158 

as  source  of  energy,  158 

as  stimulus,  158 

in  development  of  chloroj^hyH,  70 

infra-red,  71 

in  photosj'ntliesis,  70 

toxic  effect  of,  159 

ultra-violet,  71 

wave-lengths  of,  71 
Lignified  cell-wall,  100 
Lilac,  90 
Liliales,  366 
Linaria,  flower  of,  191 
Linden,  leaf  of,  63 
Linnaeus,  5,  6,  260 
Liquidamhar,  twig  of,  102 
Liriodendron,  twig  of,  96,  97,  98 


378 


INDEX 


Liverworts,  313 

Living  organisms,  as  environmental 

factors,  171 
Lycoperdales,  302 
Lycopodiales,  335 
Lycopodineae,  334 
Lycopodium,  334,  335,  336 
gametophyte  of,  336 


M 


Macrocystis,  278 

Magnesium,  importance  of,  31 

in  chlorophyll,  70 
"Male"  flower,  192 
Malpighi,  5 
Maltase,  124 
Manufacture     of    food,     in     green 

plants,  69 
Maple,  six  species  of,  258 
Marchantia,  314,  315 
Marchantiales,  313 
Maximum,  as  of  temperature,  etc., 

157 
Membrane,  cytoplasmic,  52 

osmotic,  48 

permeability  of,  49 

semipermeable,  50 
Mendel,  211,  212 

methods  of,  211 
Mendelian  ratios,  218 
Mendel's  law  of  inheritance,  7,    211- 

223 
Meristem,  142 
Mesophyll,  65,  66 
Mesophyte,  167 
Metabolism,  117-133 

constructive,  117 

destructive,  117 
Metaphase  of  mitosis,  140 
Microorganisms,  in  soil,  31 
Micropyle,  197,  199 
Mildews,  295 
Milk,  souring  of,  133 
Milkweed,  fruit  of,  200 
Mineral  nutrients,  31,  118 
Minimum,  as  of  temperature,  etc., 
157 


Mistletoe,  as  parasite,  172 
Mitosis,  139,  140,  187 

significance  of,  141 
Modern  Period,  5 

Moisture,    as  environmental   factor, 
163 

effect  of,  on  structure,  164 
Molds,  289 
Monocotyledons,  361,  364-366 

embryo  of,  198,  199,  361 

flower  of,  186,  361 

stem  structure  of,  103,  104,  361 
Monoecious,  192 
Monotropa,  174 
Morels,  294 

Morinda,  pollen  of,  186 
Morphology,  3 

experimental,  3 
Mountain  ash,  leaf  of,  64 

laurel,  flower  of,  195 
Mucorales,  290 
Mulch,  28 
Multicellular  sexual  organs,  312 

plant,  origin  of,  245 
Mushroom,  299,  300 
Mutation,  222,  223-225,  238 

in  Boston  fern,  223 

in  tobacco,  222 

origin  of  cultivated  plants  by,  224, 
225 

theory,  238 
Mycelium,  285 
Mycorrhiza,  34,  175 
Myriophylliim,    stem    structure    of, 
167 


N 


Naked  flower,  192 
Names  of  plants,  260 
Natural  Selection,  235 

objections  to  theory  of,  237 
Neck,  of  archegonium,  312 
Neck-canal  cells,  312 
Nectar,  194 
Nectary,  191,  194 
Nemalion,  282,  283 
Nereocystis,  278 


INDEX 


379 


Netted  venation,  63,  65 

Nicotiana,  flower  of,  190 

Nitrate  bacteria,  32,  33 

Nitrification,  32 

Nitrite  bacteria,  32,  33 

Nitrohacter,  32 

Nitrogen  cycle,  32,  33 

Nitrogen,  importance  of,  31,  168 

Nitrogen-fixing  bacteria,  32,  33,  175 

Nitromonas,  32 

Node,  90,  92 

Nomenclature,  260 

Nostoc,  266 

Nucellus,  197,  345,  346,  358 

Nucleus,  45,  139 

Nut,  200 

Nutrient  materials,  118 

Nym-phaea,  pollen  of,  186 


Oak,  stem  of,  105,  107 

wood  of,  110 
Objections    to    theory    of    Natural 

Selection,  237 
Oedogonium,  245,  270,  271 
Olein,  121 

One-celled  green  algae,  267 
Oogonium,  271 
Oospore,  269 
Operculum,  319,  321 
Ophioglossales,  333 
Opposite  leaf  arrangement,  91 
Optimum,   as  of  temperature,  etc., 

157 
Orchidales,  366 
Organ,  17,  246 
Organic  cycle,  118,  119 

matter  in  soil,  30 
Organisms,  in  soil,  31 
Organization,  246 
"Origin  of  Species,"  7,  235 
Orthogenesis,  238 
Oscillatoria,  266 
Osmosis,  19,  47-55 

in  growing  regions,  55 

in  the  plant  cell,  51-53 


Osmotic    movement,    of     dissolved 
substances,  48 

of  solvents,  49 

rate  of,  52 
Osmotic  pull  in  sap  ascent,  111 
Ovary,  185,  355 

Overproduction  of  offspring,  236 
Ovulate  cone,  353 
Ovule,  185,  255,  345,  346 

evolution  of,  255 
Oxidase,  129 

Oxidation,  in  respiration,  126 
Oxygen,     absorbed     in    respiration, 
126,  129 

b3'-product  of  photosynthesis,   73 


Palaeobotany,  3 

Palisade  cell,  of  leaf,  67,  68 

layer  of  leaf,  66,  67 
Palmales,  365 
Palmitin,  121 
Panicle,  193 
Papilionaceous,  363 
Pappus,  364 
Parallel  venation,  63,  65 
Paraphyses,  293 
Parasite,  171,  172,  284 
Parenchyma,  96 
Parmelia,  304 
Pasteur,  286,  287 
Pea,  germination  of,  202 

inheritance  in,  213 
Pediastrum,  269,  270 
Penicillium,  296 
Pepsin,  124 
Percolating  water,  25 
Perianth,  187 
Pericarp,  196,  199 
Peridium,  302 
Perithecium,  294 
Perisporiales,  295 

Permeability  of  membranes,  49,  50 
Peronosporales,  291 
Petal,  185,  187 
Petiole,  of  leaf.  63,  (;4 
Peziza,  293,  294 


380 


INDEX 


Pezizales,  293 
Phaeophyceae,  278 
Phaeosporales,  278 
Phallales,  302 
Phallus,  303 
Phloem,  47,  97-100 
Phoradendron,  as  parasite,  172 
Phosphorus,  importance  of,  31,  168 
Photosynthesis,  20,  69-74,  127 

by-product  of,  73 

energy  of,  70 

importance  of,  69,  128 

materials  of,  69 

mechanism  of,  70 

products  of,  72 
Photosynthetic  equation,  73 
Phototropism,  158,  159 

negative,  158 

neutral,  158 

positive,  158 
Phycocyanin,  265 
Phycoerythrin,  282 
Phycomycetes,  289 
Phyllotaxy,  90 
Phylogeny,  2,  257 
Physcia,  304 
Physiology,  3 
Pileus,  299,  301 
Pine,  cambium  of,  147 

cones  of,  352,  353 

gametophytes  of,  345,  347 

pollen  of,  186 

wood  of,  109 
Pistil,  185 
Pitcher  plant,  173 
Pith,  95,  96 
Pits  in  wood  cells,  101 
Plankton,  276 
Plant  association,  169 

foods,  117-122 

geography,  3 

kingdom,  evolution  of,  244 
size  of,  243 
survey  of,  11-16 
Plants,  compared  with  animals,  244 
Plasmolysis,  55 
Plastids,  45 
Platanus,  twig  of,  102 


Plate,  in  mitosis,  141 

Plectascales,  295 

Pleurococcus,  245,  267 

Pleurotus,  13 

Pliny  the  Elder,  4 

Plowrightia,  294 

Plumule,  198,  199 

Plurilocular  sporangium,  278,  279 

Pod,  199 

Pole,  in  dividing  cell,  141 

Pollen,  185,  186,  346 

chamber,  350 

tube,  196,  197,  347 
Pollination,  193-196 

by  insects,  194 

by  wind,  194 
Polypetalous  corolla,  190 
Polypodiuni,  327 
Polyporaceae,  301 
Polysepalous  calyx,  190 
Polysiphonia,  283 
Polytrichum,  14,  321 
Pome,  200 
Pond  scums,  275 
Pore  fungus,  301 
Porella,  316 
Postelsia,  278 

Potassium,  importance  of,  31.  168 
Potato,  starch  grains  of,  120 

tuber  of,  94 
Potential  energj^,  69,  125 
Primary  tissues,  149 
Primitive  plants,  244 
Primula,  flower  of,  195 
Procarp,  282,  283 
Proembryo,  354 
Promycelium,  297,  298 
Prophase  of  mitosis,  140 
Proserpinaca,    "amphibious"   plant 

of,  164 
Proteins,  121 

digestion  of,  124 

synthesis  of,  by  green  plants,  122 
Prothallus,  of  fern,  330,  332 
Protococcales,  267 
Protonema,  320 
Protoplasm,  19,  44,  124 
Protoplast,  44 


INDEX 


381 


Pruning,  51 

Prunus,  flower  of,  189,  362 

Pteridophyta,  12,  14,  325-342 

reasons  for  success  of,  154 
Puccinia,  297,  298 
Puff-ball,  301,  302 
Purpose,  in  plant  activities,  154 
Pyrenoid,  267,  274 
Pyrenomycetes,  294 


Quillwort,  338 


Raceme,  193,  362 
Radial  section  of  wood,  107 
Radicle,  198,  199 
Ranales,  362 
Rays,  in  wood,  107 
Receptacle,  188 

of  Fucus,  280 
Recessiveness,  213 
Red  algae,  282 

cedar,  galls  on,  169 
Reduction  division,   187,    188,    217, 
251 

of  chromosome  number,    186-188 
Regular  flower,  191 
Regulation,  154,  246 
Reproduction,  182-203 

asexual,  182,  183 

sexual,  184 

vegetative,  182 
Reproductive  organs,  20 
Reserve  cellulose,  121 
Resistance,  to  heat  and  cold,  158 
Respiration,  20,  125-133 

aerobic,  129 

anaerobic,  131 

as  index  of  activity,  129 

contrasted    with    photosynthesis, 
130 

in  every  living  cell,  129 

of  plants  and  animals,  131 
Response  to  stimulus,  155 


Rhizobium,  32 
Rhizoid,  253,  313 
Rhizome,  93,  94 
Rhizopvs,  289 
Rhodophyceae,  282 
Riccia,  313 

Ricciocarpus,  312,  313,  314 
Rice,  starch  grains  of,  120 
Rivularia,  266 
Rock  particles,  in  soil,  24 
Rockweed,  12 
Root,  19,  40-56,  325 

absorbing  region  of,  42 

as  prop,  55 

external  structure  of,  40 

fibrous,  19,  40,  41 

internal  structure  of,  46,  47 

lateral,  41 

of  climbing  plants,  55 

of  epiphytes,  55 
Root-cap,  42,  143,  144 
Root-hair,  19,  42,  43,  44 
Root-pressure,  54 

in  sap  ascent.  111 
Rootstock,  93,  94,  184 
Root-tubercles,  of  legumes,  33,  34 
Rosales,  363 
Rubiales,  364 
Runner,  184 

Run-off,  from  soil  surface,  24 
Rusts,  297 


Sabatia,  evolution  of  species  in,  232, 

233 
Saccharomycetes,  295 
Saccharomyces,  295 
Sac  fungi,  292 
Salts,  absorption  of,  53,  54 
Sambucus,  variation  in  leaves  of,  208 
Sap,  ascent  of,  111 
Sap-cavity  of  cell,  45 

development  of,  141 
Saprolegnia,  291 
Saprolegniales,  291 
Saprophyte,  173,  284 
Sap-wood,  105,  106 


382 


INDEX 


de  Saussure,  6 
Schizom.ycetes,  265 
Schizophyceae,  265 
Schizophyta,  265 
Schleidcn,  6 
Scion,  151 
Sclerenchyma,  331 
Scutellum,  198,  199 
Secondary  tissues,  149 
Sedge,  365 
Sedum,  166,  189 
Seed,  13,  198,  199,  344 

coat,  348 

development  of,  198,  345 

dispersal  of,  200,  201 

evolution  of,  254 

germination  of,  202 

origin  of,  344 

plants,  13-17,  344-366 
Seedling,  202 

Seed-production,  process  of,  196 
Segregation,  Mendelian,  213-219 
Selaginella,  336,  337 

female  gametophyte  of,  338 
Selaginellales,  336 
Self-fertilization,  198 
Self-pollination,  194 
Self-sterile,  194 
Sepal,  185,  188 
Serum,  288 
Seta,  316,  321 
Sexual  cells,  184 
Sexual  reproduction,  184 

advantages  of,  184 

origin  of,  246,  247 

organs,    multicellular,    312 
Sexuality,  in  Mucorales,  290 
Shield  fern,  14 
Shoot,  63 
Shrub,  88,  90 
Sieve-tube,  97,  98,  99 

in  vein,  68 
Sieve-plate,  98,  99 
Silo,  use  of,  137 

"Silver  grain"  of  oak  wood,  107 
Simple  leaf,  64 
Siphonales,  272 
Skunk  Cabbage,  365 


Smuts,  297 

Snapdragon,  inheritance  in,  218 

Soil,  23-35 

air  in,  29 

bacteria,  32 

crumbs,  24 

dissolved  substances  in,  30 

essential  chemical  elements  in,  31 

floccules,  24 

organic  matter  in,  30 

organisms  in,  31 

rock  particles  in,  24 

water  in,  24-29 
Solitary  flower,  193 
Solomon's  Seal,  leaf  of,  63 
Soredium,  303 
Sorus,  328,  331 
Spadix,  365 
Spathe,  365 

"Species  Plantarum,"  5,  260 
Specific  name,  260 
Spectrum,  71 

of  chlorophyll,  71,  72 
Sperm,  247,  267 
Spermagonia,  297,  298 
Spermatia,  297 

Spermatophyta,  13-17,  344-366 
Sphagnales,  319 
Sphagnum,  319,  320 
Spike,  193 

Spindle,  in  mitosis,  141 
Spirillum,  285,  286 
Spirogyra,  274,  275 
Spongy  layer,  of  leaf,  66,  67 
Sporangiophore,  289 
Sporangium,  267,  329,  331 

plurilocular,  278,  279 
Spore,  11 

Sporocarp,  333,  334 
Sporogonium,  313 
Sporophore,  300 
Sporophyll,  335,  336 
Sporophyte,  248,  250,  311 
"Sport,"  225 
Squash,  germinating  pollen  of,  197 

inheritance  in,  221 

root  growth  in,  145 

sieve  tube  of,  99 


INDEX 


383 


Squash,  variation  in  fruits  of,  209 
Stalk-ccll,  of  pollen,  347,  3.52 
StaincMi,  185 
Staminate  cone,  352 
Starch,  120 

formation  of,  from  glucose,  73 
grains,  120 
Stearin,  121 
Stem,  20,  88-112,  325 

external  structure  of,  88-93 
herbaceous,  101,  103 
internal  structure  of,  94-105 
surface  of,  91 
woody,  101,  102 
Sterigma,  296 
Sterilization,  of  sporophytic  tissue, 

316 
Stigma,  185,  347,  360 
Stimulus,  155 
Stipe,  279,  299 
Stock,  151 
Stolon,  183,  184 

of  Rhizopus,  289 
Stoma,  65,  66 
Stonecrop,  flower  of,  189 
Strawberry,  asexual  reproduction  of, 

183 
Striations,  of  starch  grains,  120 
Strobilus,  335,  336 
Struggle  for  existence,  236,  237 
Style,  185 
Substratum,  285 
Sucrose,  119 
Sugars,  119 

digestion  of,  123 
fermentation  of,  131 
Sulphur,  importance  of,  31 
Sundew,  173 

Survival  of  the  fittest,  236,  237 
Suspensors,  of  Mucorales,  290 
Swamp  association,   170 
Sweet  gum,  twig  of,  102 
Sycamore,  twig  of,  102 
Symbiosis,  175,  303 
Sympetalae,  362,  364 
Symplocarpus,  365 
Synergids,  360 
Synthesis,  of  carbohydrates,  73 


Synthesis,  of  proteins,  122 
Syringa,  98 
Systematic  botany,  2 


T 


Tangential  section  of  wood,  106,  107 
Tap-root,  19,  41 
Taraxacum,  pollen  of,  186 

variation  in,  210 
Taxonomy,  2 
Teleutospore,  297,  298 
Telophase,  of  mitosis,  140 
Temperature,  as  environmental  fac- 
tor, 156 
maximum,  157 

minimum,  157 

optimum,  157 

of  plant  body,  157 
Terminal  growing-point,  143,  144 
Tetrads,  of  spores,  313 
Tetraspore,  283,  284 
Tetrasporic  plant,  283,  284 
Thallophyta,  11,  264-305 
Thallus,  264 
Theophrastus,  4 
Tissue,  19,  246 
Toadflax,  flower  of,  191 
Tobacco,  flower  of,  190 

mutation  in,  222 
Tolerance,  of  shade,  161 
Tooth  fungi,  302 
Toxin,  288 

Tracheal  cell,  100,  101 
Tracheid,  100,  101,  350 

in  vein,  68 
Translocation,  of  food,  112 
Transpiration,  19,  74-78 

as  cooling  process,  77 

rate  of,  75 

significance  of,  77 

stream,  77 
Transverse  section  of  wood,  106,  107 
Tree,  88,  91 
Trichogyne,  283,  284 
Trillium,  flower  of,  186 
Trophophyte,  168 


384 


INDEX 


True  ferns,  327 

molds,  290 

mosses,  320 
Truffles,  294 
Trunk,  88 
Trypsin,  124 
Tryptophane,  122 
Tube-nucleus,  196,  347 
Tuber,  93,  94,  184 
Tuberales,  294 
Tubiflorales,  364 
Tubular  algae,  272 
Tulip-tree,  twig  of,  96,  97,  9 
"Tumble  weeds,"  201 
Turgidity,  of  plant  cells,  55 


Ulothrix,  270,  271 
Umbel,  193,  363 
Umbellales,  363 
Unisexual  flower,  192 
Unit  characters,  212 
Uredinales,  297 
Uredospore,  297,  298 
Ustilaginales,  297 


Vaccination,  288 
Vaccinium,  flower  of,  190 
Vacuole,  of  cell,  45,  141 
Valves,  of  diatoms,  276 
Variation,  208,  209 

in  floral  parts,  188 

produced  by  environment,  210 
Vascular  plants,  326 
Vaucheria,  273 
Vegetative  reproduction,  182 

structures  and  functions,  182 
Veins,  of  leaf,  63,  65,  66,  68 
Venation  of  leaf,  63,  65 
Venter  of  archegonium,  312 
Ventral  canal-cell,  312 
Venus 's  Fly  Trap,  173 
Vessel,  101,  356 


Vessel-cell,  100,  101 

Vestigial  organs,  231 

Volva,  299 

Volvox,  269 

de  Vries,  theory  of,  238 

W 

Water,  absorption  of,  53,  54 

capillary,  25 

rise  of,  in  soil,  27,   28 

gravitational,  25 

hydrostatic,  25 

hygroscopic,  29 

importance  of,  24,  74,  163 

in  soil,  24 

movement  of,  in  soil,  26 

percolating,  25 
Water  columns,  cohesive  power  of, 

112 
Water  ferns,  333 
Water  films,  26 
Water-holding  capacity,  26 
Water-lily,  166 
Water-milfoil,  166 
Water  molds,  291 
Water  table,  25 

Water-requirement  of  plants,  76 
Wave-lengths,  of  light,  71 

used  in  photosynthesis,  72 
Wheat,  rust  of,  298 

starch  grains  of,  120 
Whorl,  91 
Wild  carrot,  363 
Willow,  flowers  of,  192 
Wind-pollination,  193,  194 
Wood,  100,  105-111 

differences  in,  108 

oak,  110 

of  root,  47 

of  stem,  100 

pine,  108,  109 

three  planes  of  section  of,  106 
Wood-fiber,  100,  101 
Wood-parenchyma,  100,  101 
Wood-ray,  100,  101,  107 


INDEX 


385 


Xanthophyll,  70 
Xerophyte,  165,  166,  170 

leaf  blade  of,  165 

stoma  of,  165 
Xylem,  47,  100 

Y 

Yeast,  fermentation  by,  131 
Yeasts,  295 


Zein,  121 
Zoospore,  267 
Zygospore,  26S 

formation  of,  in  Mucor,  290 
Zygote,  184,  246 
Zymase,  131 


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